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Lesson · Grade 7 · Mathematics

Estimating to Check a Decimal Answer

  • Grade 7
  • Mathematics
  • 1 min 12 s
  • British Columbia
  • BC7-MA-22

To check the claim 4.8 × 3.1 = 148.8, the video rounds each number to the nearest whole number to get an estimate of 5 × 3 = 15, and a number line shows that 148.8 is about 10 times too big. Moving the decimal point one place left gives 14.88, which lands right beside 15, so 4.8 × 3.1 = 14.88, and the video ends on the recap: round, estimate, then compare.

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In one sentence

The question this lesson answers

Someone worked out 4.8 times 3.1 and got 148.8.

We can check whether that makes sense with a quick estimate.

Transcript

What the lesson says

Someone worked out 4.8 times 3.1 and got 148.8.

We can check whether that makes sense with a quick estimate.

First, round 4.8 to the nearest whole number, which is 5.

Then, round 3.1 to the nearest whole number, which is 3.

Now 5 times 3 is 15, so the answer should be close to 15.

On a number line, 148.8 is nowhere near 15.

In fact, 148.8 is about 10 times too big.

The digits are right, but the decimal point is in the wrong place.

Move the point one place to the left, and 148.8 becomes 14.88.

Now 14.88 is very close to 15, so this answer makes sense.

So 4.8 times 3.1 is 14.88.

To check a decimal answer, round the numbers, estimate, and compare.

Curriculum

Where this fits

British Columbia, Grade 7

Mathematics, Number: operations with decimalsBC7-MA-22

Previous: Order of Operations with Decimals and BracketsNext: Which is bigger, 0.5 or 0.45?

The outcomes and strands follow the British Columbia curriculum.

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