Question
By what number should $(-20)^{-1}$ be divided to obtain $(-10)^{-1}$ ?

Answer

Let the required number be $x .
$(-20)^{-1} \div x =(-10)^{-1}$ 
$\Rightarrow \frac{1}{(-20)} \times \frac{1}{ x }=\frac{1}{(-10)}$
$\Rightarrow \frac{1}{(-20 x )}=\frac{1}{(-10)}$
$\therefore x =\frac{(-10)}{(-20)}=\frac{1}{2}=2^{-1}$
 Hence, the required number is $2^{-1}$.

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