Lesson 3: Estimate: Use Rounding or 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 $48 \times 72$, round $48$ to the nearest ten, which is $50$. Then, calculate $50 \times 70 = 3500$. So, $48 \times 72 \approx 3500$.

• To estimate $32 \times 88$, round $32$ down to $30$ and round $88$ up to $90$. Then, calculate $30 \times 90 = 2700$. So, $32 \times 88 \approx 2700$.

Property

To estimate a product $a \times b$, replace the factors with nearby compatible numbers, $c$ and $d$, such that $c \approx a$ and $d \approx b$. The estimate is the product $c \times d$.

Examples