Prove the following
Question:

$(213 \times 657)^{-1}=213^{-1} \times$________.

Solution:

Let us assume the missing number be $x$ Then,

$=1 /(213 \times 657)=(1 / 213) \times(x)$

$X=213 /(213 / 657)$

$X=1 / 657$

$X=657^{-1}$

So, $(213 \times 657)^{-1}=213^{-1} \times \underline{657^{-1}}$