By what rational number should $\dfrac{25}{8}$ be multiplied to get $\dfrac{-20}{7}$ ?

Let the required number be x. Then,

$\dfrac{25}{8} \times x = \dfrac{-20}{7} \\[1em] \Rightarrow x = \dfrac{-20}{7} \div \dfrac{25}{8} \\[1em] = \dfrac{-20}{7} \times \dfrac{8}{25} \quad \left[\text{Reciprocal of } \dfrac{25}{8} \text{ is } \dfrac{8}{25}\right] \\[1em] = \dfrac{-4}{7} \times \dfrac{8}{5} \quad \text{[Dividing 20 and 25 by 5]} \\[1em] = \dfrac{-4 \times 8}{7 \times 5} \\[1em] = \dfrac{-32}{35}$

The required number is $\dfrac{-32}{35}$.