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Add and subtract decimals fluently

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5106806
A hiker carries four packages with masses of \(0.85\,\text{kg}\), \(2.14\,\text{kg}\), \(3.15\,\text{kg}\), and \(1.86\,\text{kg}\). Explain how to find the total mass mentally by making convenient pairs. Then give the total.

Hints

- Look closely at the hundredths digits. - Which pairs add to a whole number? - You do not need to add the four numbers in the order given.

Solution

1. Pair numbers that add to whole numbers: \(0.85 + 3.15\) and \(2.14 + 1.86\). 2. Evaluate the first pair: \(0.85 + 3.15 = 4.00\). 3. Evaluate the second pair: \(2.14 + 1.86 = 4.00\). 4. Add the pair sums: \(4.00\,\text{kg} + 4.00\,\text{kg} = 8.00\,\text{kg}\).

Answer

The total mass is \(8.00\,\text{kg}\). One efficient grouping is \((0.85 + 3.15) + (2.14 + 1.86) = 4.00 + 4.00 = 8.00\).
5113356
Evaluate each expression efficiently by rearranging or grouping the decimal numbers. a) \(12.35 + 7.8 + 7.65 + 2.2\) b) \(24.7 - 3.9 - 6.1 - 4.7\)

Hints

- Look for decimal parts that combine to make whole numbers. - Several subtractions can be rewritten as subtracting the sum of the amounts. - In b), find two numbers with the same tenths digit and two numbers whose decimal parts make a whole.

Solution

1. For a), group numbers that add to whole numbers: \((12.35 + 7.65) + (7.8 + 2.2) = 20 + 10 = 30\). 2. For b), rewrite the subtractions as subtracting a total and group conveniently: \((24.7 - 4.7) - (3.9 + 6.1) = 20 - 10 = 10\).

Answer

a) \(30\) b) \(10\)
5122356
Evaluate the expression mentally by grouping the decimal numbers efficiently. \(12.4 - 5.8 - 2.4 - 1.2\)

Hints

- Look for decimal parts that combine or cancel to make whole numbers. - Rewrite repeated subtraction as subtracting a sum before regrouping. - Keep the signs attached to the terms when you rearrange.

Solution

1. Group the terms to create easy differences and sums: \((12.4 - 2.4) - (5.8 + 1.2)\). 2. Evaluate the groups: \(12.4 - 2.4 = 10\) and \(5.8 + 1.2 = 7\). 3. Subtract: \(10 - 7 = 3\).

Answer

\(3\)
5106466
A hiker starts with \(1.5\) liters of water in a bottle. During the hike, the hiker drinks \(\frac{3}{8}\) liter, adds \(0.5\) liter at a spring, and then drinks another \(0.4\) liter. a) How much water is in the bottle at the end? b) What is the minimum bottle capacity needed so that it does not overflow when refilled?

Hints

- Decide whether fractions or decimals will be easier to use. - Subtract when water is consumed and add when water is refilled. - For the capacity, identify the greatest amount in the bottle at any time.

Solution

1. Convert \(\frac{3}{8}\) to a decimal: \(\frac{3}{8}=0.375\). 2. The final amount is \(1.5-0.375+0.5-0.4=1.225\) liters. 3. After the first drink, the bottle contains \(1.5-0.375=1.125\) liters. After refilling, it contains \(1.125+0.5=1.625\) liters. This is the greatest amount in the bottle. 4. Therefore, the minimum capacity is \(1.625\) liters.

Answer

a) \(1.225\) liters b) At least \(1.625\) liters
5351996
Read the decimals represented by points \(A\) and \(B\). Then find the distance between the two points.
Figure for problem 535199

Hints

- Determine the value of one small interval between consecutive tenths. - Count hundredths from the nearest labeled tenth. - Subtract the smaller coordinate from the larger one.

Solution

1. Each tenth is divided into \(10\) equal intervals, so each small interval represents \(0.01\). 2. Point \(A\) is four intervals right of \(5.2\), so \(A=5.24\). 3. Point \(B\) is seven intervals right of \(5.4\), so \(B=5.47\). 4. The distance is \(5.47-5.24=0.23\).

Answer

\(A=5.24\), \(B=5.47\), and the distance is \(0.23\).

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