WAEC 2020 · Paper 1 · Q6

Given that $x : \mathbb{R} \to \mathbb{R}$ is defined by $x = \dfrac{y + 1}{5 - y}$ , $y \in \mathbb{R}$ , find the domain of $x$ .

Worked solution (try it first)

A fraction is undefined when its denominator is zero, so find where $5 - y = 0$ .
That happens at $y = 5$ , so every other real $y$ is allowed.
The domain is $\{y : y \in \mathbb{R}, y \neq 5\}$ , option C.