- Rewrite the equation with an explicit constant term. - Evaluate the output at zero input.

1. The missing constant term is \(0\), so the \(y\)-intercept is \(0\). 2. Therefore, the model predicts \(y=0\) when \(x=0\).

The intercept is \(0\), so the model predicts \(y=0\) at \(x=0\).

- Look for a pair where larger values of the first quantity tend to accompany smaller values of the second. - Eliminate contexts in which the two quantities naturally increase together.

1. A negative fitted slope means the predicted output tends to decrease as the input increases. 2. As outside temperature rises, home-heating fuel use generally falls, so choice b) is reasonable.

b) Outside temperature and home-heating fuel used.

- Use only information supplied by the constant term. - Do not infer a slope from the intercept.

1. The intercept is the model's predicted value of \(y\) when \(x=0\). 2. Therefore, the model predicts \(y=28\) when \(x=0\); the intercept alone does not determine the direction or rate of change.

The model predicts \(y=28\) when \(x=0\).

- Pick two easy-to-read points on the straight line itself. - Compare their vertical change with their horizontal change.

1. On the line, moving from \(x=1\) to \(x=2\) changes \(y\) from \(4\) to \(6\). 2. The increase in predicted \(y\) is \(2\) per \(1\) unit of \(x\).

\(2\) units of \(y\) for each \(1\) unit of \(x\).

- Find the vertical axis, where x is zero. - Read the height at which the straight line meets that axis.

1. The y-intercept is where the line reaches \(x=0\). 2. On the graph, it crosses the vertical axis at \(y=8\).

\(8\).

- Find how much the line rises over an easy number of hours. - Divide the change in cost by the change in time. - Check that the units match a cost per hour.

1. On the fitted line, the cost rises from about \(\$10\) at \(1\) hour to about \(\$20\) at \(3\) hours. 2. The cost rises \(\$10\) in \(2\) hours, giving slope \(\$5\) per hour. 3. The predicted cost rises about \(\$5\) for each additional rental hour.

About \(\$5\) per hour. Each extra hour increases the predicted rental cost by about \(\$5\).

- Identify the units of the vertical change and the horizontal change. - Use association language because the equation is a fitted model.

1. The slope is \(4.5\) score points per hour. 2. In a fitted model, slope describes an associated average change. 3. Each additional hour of weekly practice is associated with about \(4.5\) more score points.

Each additional hour of weekly practice is associated with an increase of about \(4.5\) score points.

- Interpret the model’s output when the input equals zero. - Because this is a fitted model, describe the intercept as a prediction rather than an exact pricing rule.

1. The intercept is the model’s predicted cost when \(d=0\). 2. Substituting \(d=0\) gives \(C=6\). 3. The model predicts a delivery cost of about \(\$6\) at zero miles, which may represent a base charge.

The model predicts a cost of about \(\$6\) when the delivery distance is \(0\) miles; in context, this may represent a base charge.

- Use the sign to determine whether the predicted output rises or falls. - Attach output units per input unit to the slope.

1. The slope is \(-3.2\) cups per degree Fahrenheit. 2. The negative sign indicates a decrease as temperature increases. 3. Each \(1\,\text{°F}\) increase is associated with about \(3.2\) fewer cups sold.

Each \(1\,\text{°F}\) increase in temperature is associated with about \(3.2\) fewer cups of soup sold.

- Separate a rate from a total predicted amount. - Look at the role of both terms in the equation.

1. The slope is a rate of change, not an output value by itself. 2. It compares a \(7\)-unit change in \(y\) with a \(1\)-unit change in \(x\). 3. The predicted \(y\)-value also depends on \(x\) and the intercept \(18\).

The slope means the predicted \(y\)-value increases by \(7\) units for each \(1\)-unit increase in \(x\); it does not mean \(y=7\).

- Form a “vertical units per horizontal units” rate. - The intercept lies on the vertical axis.

1. Slope units are output units divided by input units. 2. The slope is measured in meters per minute. 3. The intercept is an output value, so it is measured in meters.

Slope: meters per minute. Intercept: meters.

- Compare like parameters first. - Parallel model lines differ by a constant vertical amount.

1. Both slopes are \(2.5\), so both models predict the same change for each unit increase in \(x\). 2. Model \(A\) has intercept \(18\), and model \(B\) has intercept \(11\). 3. Model \(A\) predicts values \(7\) units higher for every value of \(x\).

The models have the same rate of change. Model \(A\) starts \(7\) units higher and predicts \(7\) more units for every value of \(x\).

- Compare the two slope coefficients. - Subtract the smaller rate from the larger rate.

1. The intercepts are equal, so both models predict \(30\) when \(x=0\). 2. Model \(B\) has the larger slope because \(2.1>1.2\). 3. Its predictions grow by \(2.1-1.2=0.9\) unit more for each unit increase in \(x\).

Model \(B\), by \(0.9\) unit more for each unit increase in \(x\).

- Do not decide both questions from one coefficient. - Match “starting” with one parameter and “rate” with the other.

1. The intercepts are \(9\) for \(P\) and \(15\) for \(Q\), so \(Q\) starts higher. 2. The slopes are \(4\) for \(P\) and \(3\) for \(Q\), so \(P\) increases faster. 3. The two comparisons involve different parameters.

Model \(Q\) has the larger starting value; model \(P\) has the larger rate of increase.

- Translate the fitted value at zero input into the context. - Check whether the measurements began at the starting reference point.

1. The intercept is the model’s predicted distance when time is \(0\). 2. The fitted line predicts \(d=0\) at \(x=0\). 3. If measurements begin when the object starts moving from the reference point, zero distance at zero elapsed time is meaningful.

The zero intercept means the fitted line predicts \(0\) distance traveled when no time has elapsed, which matches starting at the reference point.

- What happens to the formula's value when you increase \(b\) by 1? - What number would the formula give if \(b=0\)? Was that number measured?

1. Increasing \(b\) by \(1\) increases the predicted cost by \(\$3\). That is the slope, not \(\$6\). 2. At \(b=0\), the model gives \(C\approx 6\); this is the predicted base cost. 3. Since zero-bag orders were not recorded, the \(\$6\) value is an estimate, not a measured charge.

The model adds about \(\$3\) for each bag, not \(\$6\). The \(\$6\) is the predicted cost at zero bags, outside the recorded data.

- What does the constant number in the fitted equation predict? - Where is \(0\) degrees compared with the recorded temperature range?

1. The line's y-intercept is \(30\), so the model predicts \(30\) cones when \(x=0\). 2. The prediction is not impossible, but \(0\) degrees is far outside the observed range of \(70\) to \(90\). 3. The fit does not give reliable evidence about such a cold day.

No. The line predicts \(30\) cones at zero degrees, but that temperature is far outside the data range, so the prediction is not reliable.

- Use points on the line that fall exactly on readable grid locations. - Compute vertical change over horizontal change in left-to-right order.

1. Two clear points on the line are \((0, 12)\) and \((6, 3)\). 2. The slope is \(\frac{3-12}{6-0}=\frac{-9}{6}=-1.5\). 3. The fit line has slope \(-1.5\).

The slope is \(-1.5\).

- Attach score-point units per workshop to the slope. - Read the observed point and the fitted line at the same \(x\)-value. - Compare observed minus predicted to describe the vertical difference.

1. The slope is \(2.8\) score points per workshop. 2. Each additional workshop is associated with about \(2.8\) more skill-score points. 3. At \(x=5\), the model predicts \(y=2.8\cdot 5+14=28\). 4. The plotted observation is \((5, 31)\). 5. The observation is \(31-28=3\) points above the model prediction.

a) Each additional workshop is associated with about \(2.8\) more skill-score points. b) The model predicts \(28\). The observed score is \(31\), which is \(3\) points above the prediction.

- Attach percentage-point units per hour to the slope. - For the target charge, set the model output equal to \(42\). - Solve the resulting equation for the input \(x\).

1. The slope is \(-6\) percentage points per hour. Each additional hour of use is associated with a predicted decrease of \(6\) percentage points in charge. 2. Set the model output equal to \(42\): \(42=-6x+96\). 3. Subtract \(96\): \(-54=-6x\). 4. Divide by \(-6\): \(x=9\).

a) Each additional hour of use is associated with a predicted decrease of \(6\) percentage points in battery charge. b) \(9\) hours.

- Check whether zero input was represented in the data. - Separate the equation’s intercept from a trustworthy real-world conclusion.

1. The intercept is the model’s value at \(a=0\), so it predicts a height of \(52\) inches. 2. Age \(0\) is far outside the observed range \(8\) to \(14\). 3. The algebraic intercept is \(52\) inches, but interpreting it as a reliable birth height is an unsupported extrapolation.

No. The intercept predicts \(52\) inches at age \(0\), far outside the data range, so it is not a reliable estimate of birth height.

- Translate the intercept into a zero-input statement. - Check whether that zero-input situation can occur in the population.

1. The intercept predicts height when shoe length is \(0\,\text{cm}\). 2. A shoe length of \(0\,\text{cm}\) is not meaningful for the people represented. 3. The intercept locates the fitted line mathematically but lacks a useful literal meaning in context.

The intercept predicts height at an impossible shoe length of \(0\,\text{cm}\), so it is mainly a mathematical feature of the fit.

- Choose two readable points on the drawn line, not two data points. - Divide the vertical change by the horizontal change and include units. - Use the slope to scale the predicted change to \(7\) minutes.

1. Two convenient points on the line are \((0, 4)\) and \((5, 12)\). 2. The slope is \(\frac{12-4}{5-0}=\frac{8}{5}=1.6\) liters per minute. 3. The model associates each additional minute with an increase of about \(1.6\) liters in the predicted amount. 4. Over \(7\) minutes, the predicted increase is \(1.6\cdot 7=11.2\) liters.

a) The slope is about \(1.6\) liters per minute, meaning the predicted amount increases by about \(1.6\) liters for each additional minute. b) About \(11.2\) liters.

- In each equation, identify the coefficient of \(x\). - State the slope as a predicted change in score for one additional practice hour. - Compare how each entire line sits within the point cloud rather than checking only one point.

1. Arjun's slope is \(2.2\), so that model associates each additional practice hour with an increase of about \(2.2\) score points. 2. Elena's slope is \(2.5\), so that model associates each additional practice hour with an increase of about \(2.5\) score points. 3. A hand-drawn fit line summarizes a scattered cloud; it is not forced through every observation. 4. Both lines follow the upward trend and stay close to the center of the cloud, so their slightly different slopes are both defensible.

a) Arjun's model predicts about \(2.2\) more score points per additional practice hour. Elena's model predicts about \(2.5\) more score points per additional practice hour. b) Both lines are reasonably centered in the scattered data, so informal fitting can produce slightly different defensible slopes.

- Compare the requested input with the data range. - Consider whether the context supports indefinite linear growth.

1. Day \(100\) is far outside the observed range. 2. The fitted linear pattern may not continue for that long. 3. Biological growth can level off, so the extrapolated prediction may be unreasonable.

The \(138\,\text{cm}\) prediction is a distant extrapolation beyond days \(5\) through \(25\), where the linear trend may no longer hold.

- Translate the intercept into a prediction at zero liquid volume. - Check whether that predicted mass is physically possible.

1. The intercept predicts \(m=-12\,\text{g}\) when \(v=0\,\text{mL}\). 2. This would mean the empty container has a negative mass, which is physically impossible. 3. The intercept likely comes from extending the fitted line beyond the useful data range, so it should not be interpreted literally.

The intercept predicts a mass of \(-12\,\text{g}\) for the empty container. Because negative mass is impossible, the intercept is not meaningful in context.

- Use the two line points to find vertical change per horizontal change. - Substitute either point into \(y=mx+b\) after finding \(m\). - Interpret the slope as a predicted weekly change and the intercept as the prediction at week \(0\).

1. The slope is \(\frac{23-11}{9-3}=\frac{12}{6}=2\) centimeters per week. 2. Substitute \((3, 11)\) into \(y=2x+b\): \(11=2\cdot 3+b\), so \(b=5\). 3. The fit equation is \(y=2x+5\). 4. The slope associates each additional week with about \(2\) more predicted centimeters, and the intercept predicts a height of \(5\) centimeters at week \(0\).

\(y=2x+5\). The slope is \(2\) centimeters per week, and the intercept predicts \(5\) centimeters at week \(0\).

- Read the slope units as “vertical units per horizontal unit.” - The intercept has the vertical variable's units.

1. Slope units are vertical-variable units per horizontal-variable unit. 2. Dollars per item means the vertical variable is measured in dollars and the horizontal variable is measured in items. 3. Therefore, total cost must be on the y-axis.

Total cost must be on the y-axis because the slope and intercept both use dollars as the output unit.

- Treat the two coefficients as answering different contextual questions. - For the prediction, substitute the stated input into the full equation.

1. The slope means each additional reading hour is associated with about \(3.6\) more predicted score points. 2. The intercept predicts a score of \(41\) at \(0\) reading hours; its usefulness depends on whether \(0\) hours is within the observed data range. 3. At \(h=6\), \(s=3.6\cdot 6+41=62.6\).

a) About \(3.6\) predicted score points per additional reading hour. b) A predicted score of \(41\) at \(0\) hours, interpreted cautiously if \(0\) was not observed. c) \(62.6\) points.

- Replace exact and causal language with statistical-model language. - Interpret the negative slope as a decrease in the predicted response.

1. A fitted model describes an association, not a proven causal effect. 2. Its slope gives an approximate change in the predicted response, not an exact change for every student. 3. The negative slope should be described as about \(2.2\) fewer predicted errors per additional practice hour.

Each additional practice hour is associated with about \(2.2\) fewer predicted errors.