Lesson 1: Estimating Products Using Rounding and Compatible Numbers

Property

To estimate the product of two numbers, round one or both factors to a nearby place value (like the nearest ten or hundred) to make the multiplication easier to perform mentally.
The symbol $\approx$ means "approximately equal to".

Examples

• To estimate $48 \times 7$, round $48$ to the nearest ten, which is $50$. Then, calculate $50 \times 7 = 350$. So, $48 \times 7 \approx 350$.

• To estimate $615 \times 4$, round $615$ to the nearest hundred, which is $600$. Then, calculate $600 \times 4 = 2400$. So, $615 \times 4 \approx 2400$.

Property

An overestimate is an estimated product that is greater than the actual product. An underestimate is an estimated product that is less than the actual product. The direction you round or adjust the factors helps determine if the estimate is an over- or underestimate.

Examples