Multiply decimals using whole-number facts
Use a whole-number fact, then account for the size of each decimal. A quick size check helps catch a misplaced point.
A rectangle is 0.6 metre wide and 0.7 metre high. You want its area. Since both sides are shorter than a metre, 42 square metres cannot be right. The known fact 6 × 7 = 42 still helps, but now you are counting hundredths of a square metre.
Six tenths of the width, seven tenths of the height
The overlap fills forty-two of one hundred equal squares. The large square stands for one square metre.
Count the small squares
Find the area: 0.6 × 0.7 square metres.
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Use the whole-number fact
The grid has six columns and seven rows in the overlap. That makes forty-two small squares.
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Name the size of those squares
Each small side is one tenth of a metre. A tenth times a tenth makes a hundredth of a square metre.
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Check against the starting size
Taking seven tenths of 0.6 must give less than 0.6. The answer passes that size check.
Why counting decimal places works
Each original number was ten times smaller than the whole-number version. Their product is one hundred times smaller. That is why the answer needs two decimal places in all.
For 1.2 × 0.03, use 12 × 3 = 36. One number has one decimal place and the other has two. Scale down by a thousand to get 0.036.
Multiplying by a positive number below one makes an amount smaller. Multiplying by a number above one makes it larger. Use this to check where the point belongs.
Try it yourself
Work out an answer and explain your reasoning. Then check the explanation.
Restore a different scale
Find the product. Decide where the point belongs and check its size.
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Four times eight is thirty-two. Three decimal places make that thirty-two thousandths.
Check the answer's size
This product looks too large. Find the correct value and explain the size check.
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Twenty-five times four is one hundred. Two decimal places turn this into 1.00.
Put the decimal point to the test
Use a whole-number fact, account for both decimals and check the size. The game gives new pairs, so choose the point rather than copying the example.
2-minute practice · keep going after mistakes · opens in a new tab