The time-height graph shows a 60-minute hot-air-balloon flight.
a) Describe the flight during \([0, 20]\), \([20, 40]\), and \([40, 60]\), with time measured in minutes.
b) Find and interpret the average rate of change of the height, in meters per minute, over each interval.
c) An observer claims, “Because the average upward velocity over \([0, 20]\) is \(15\,\frac{\text{m}}{\text{min}}\), the balloon must have been at \(250\,\text{m}\) after exactly \(10\) minutes.” Evaluate both the numerical conclusion and the reasoning.

Hints
- Identify what each axis and unit represent.
- Use the slope between the endpoint points of each interval.
- What does a negative vertical rate mean?
- Does an average velocity have to equal the velocity at every moment?
- Can you assume a curved graph is linear between its endpoints?
Solution
1. From \(0\) to \(20\) minutes, the balloon rises from \(100\,\text{m}\) to \(400\,\text{m}\), first slowly, then more quickly, and then more slowly. From \(20\) to \(40\) minutes, it remains at \(400\,\text{m}\). From \(40\) to \(60\) minutes, it descends to \(100\,\text{m}\), with a changing descent rate.
2. Over \([0, 20]\), the average rate is \(\frac{400 - 100}{20} = 15\,\frac{\text{m}}{\text{min}}\). Over \([20, 40]\), it is \(\frac{400 - 400}{20} = 0\,\frac{\text{m}}{\text{min}}\). Over \([40, 60]\), it is \(\frac{100 - 400}{20} = -15\,\frac{\text{m}}{\text{min}}\).
3. The graph does show \(h(10) = 250\,\text{m}\), so the numerical conclusion happens to be correct. However, using the average rate to assume constant linear motion is invalid because the graph is curved. The agreement occurs because of the shape and symmetry of this particular graph.
Answer
a) \([0, 20]\): the balloon rises from \(100\,\text{m}\) to \(400\,\text{m}\) at a changing rate.
\([20, 40]\): it stays at \(400\,\text{m}\).
\([40, 60]\): it descends from \(400\,\text{m}\) to \(100\,\text{m}\) at a changing rate.
b) \([0, 20]\): \(15\,\frac{\text{m}}{\text{min}}\)
\([20, 40]\): \(0\,\frac{\text{m}}{\text{min}}\)
\([40, 60]\): \(-15\,\frac{\text{m}}{\text{min}}\)
c) The graph gives \(h(10) = 250\,\text{m}\), but the observer’s reasoning is invalid because an average velocity does not imply constant velocity.