By what number should

Using the property a−1 = 1/a for every natural number a, we have (−15)−1 = −1/15 and (−5)−1 = −1/5. We have to find a number x such that

$\frac{\frac{-1}{15}}{\frac{x}{1}}=\frac{-1}{5}$

or $\frac{-1}{15} \times \frac{1}{\mathrm{x}}=\frac{-1}{5}$

or $x=\frac{1}{3}$

Hence, $(-15)^{-1}$ should be divided by $\frac{1}{3}$ to obtain $(-5)^{-1}$.