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Subtract decimals: FAQ

Frequently asked questions about subtracting decimals.

How do we estimate with subtracting decimals?

Estimating means making a guess that is close to the exact answer, but not exact. Estimating can help us check if our answer makes sense, or if we made a mistake. To estimate with subtracting decimals, we can round each decimal to the nearest whole number, and then subtract the rounded numbers. For example, if we want to estimate 12.344.56, we can round 12.34 to 12 and 4.56 to 5, and then subtract 125=7. This means that our exact answer should be close to 7.
Try it yourself with this exercise:

How do we subtract decimals visually?

We can subtract decimals visually by using models like grids, number lines, or base-ten blocks. These models can help us see what the decimals mean and how they relate to each other. For example, if we want to subtract 0.40.2, we can use a grid that is divided into 10 equal parts, and shade in 4 parts to represent 0.4, and then unshade 2 parts to represent 0.2. Then we can count how many parts are left shaded, and that is our answer.
Try it yourself with this exercise:

How do we subtract decimals and whole numbers that have different number of decimal places?

To subtract decimals and whole numbers that have different number of decimal places, we can use zeros as placeholders to make them have the same number of decimal places. Then, we can use a place value chart to subtract each place value column.
Try it yourself with this exercise:

Why do we need to learn about subtracting decimals?

Subtracting decimals is useful for many situations in life, such as:
  • Measuring lengths, weights, volumes, temperatures, or time. For example, if you run a lap in 2.35 minutes and your friend runs it in 2.42 minutes, how much faster are you? To find out, you need to subtract 2.42 from 2.35, which involves subtracting decimals.
  • If you receive an allowance and are saving up for a specific toy or item, you might need to subtract the cost of the item from the amount they have saved to see how much more they need to save up.
  • If you're baking and need to measure out ingredients, they might need to subtract a certain decimal amount (e.g. 0.75 cups) from the total amount you have on hand to make sure you have enough left for another recipe.
  • If you're tracking the amount of time they spend on different activities (for example, homework, playing outside, watching TV), you might need to subtract two decimal time measurements to figure out how long you spent on a particular activity.

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