By what number should we multiply

Let the other number be $\mathrm{x}$.

$\therefore \mathrm{x} \times \frac{-15}{28}=\frac{-5}{7}$

or $\mathrm{x}=\frac{-5}{7} \div \frac{-15}{28}$

or $\mathrm{x}=\frac{-5}{7} \times \frac{28}{-15}$

or $\mathrm{x}=\frac{4}{3}$

Thus, the other number is $\frac{4}{3}$.