Question

When $0.000315$ is multiplied by $7{,}928{,}564$ the product is closest to which of the following?

$210$

$240$

$2{,}100$

$2{,}400$

$24{,}000$

Round each factor to one significant digit.

Think of $0.000315$ as about $3\cdot10^{-4}$ and $7{,}928{,}564$ as about $8\cdot10^6$ .

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We can approximate the product as $\begin{align*} (3 \cdot 10^{-4})(8 \cdot 10^6) &= 24 \cdot 10^2 \\ &= 2{,}400. \end{align*}$

Thus, D is the correct answer.