NECO 2022 · Paper 1 · Q33

If $M$ varies inversely as $N$ and $M = \frac{15}{2}$ when $N = 4$ , find the value of $N$ when $M = \left(\frac{1}{20}\right)^{-1}$ .

Worked solution (try it first)

Write $M = \dfrac{k}{N}$ , so $k = MN = \frac{15}{2} \times 4 = 30$ .
A power of $-1$ turns the fraction upside down: $M = \left(\frac{1}{20}\right)^{-1} = 20$ .
So $N = \dfrac{k}{M}$
$= \dfrac{30}{20}$
$= 1\frac{1}{2}$ , option C.