Learn how to estimate calculations

Estimations are used to quickly estimate the answer of an expression.

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Example

$$134+159\approx 130+160\approx 290$$

You look at the last digit in each number

If the last number is < 5, then round off to the nearest 0

If the last number is ≥ 5, then round off to the nearest 10

You can estimate products and quotients as well.

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Example

Round off the factors and multiply the factors.

$$\frac{55.8}{7.42}\to\frac{56}{7}=8$$

$$55.8\approx 56$$

$$7.42\approx 7$$

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Example

Multiply the two decimal numbers.

First we multiply the two numbers without estimating this to be able to compare the different results.

$$\begin{matrix} 3.5\cdot 7.3= x\\ \\ \, \: \, \, \, \, \, 3.5\\ \,\: \, \underline{\cdot \, \, 7.3}\\ \, \, \: \, \, \, 105\\ \underline{+2450}\\ \: \, 25.55\\ \end{matrix}$$

Now if we would have estimated the answer instead we would have round off 3.5 to 4 and 7.3 to 7. This would have given us an estimate of

$$3.5\cdot 7.3\approx 4\cdot 7=28$$

As we can see the answers are pretty similar, but not identical.

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Video lesson

Estimate