Tick (✓) the correct answer:

(a) 50

Let $x$ be the required number. Then, we have:

$20 \%$ of $x+40=x$

$\Rightarrow\left(x \times \frac{20}{100}\right)+40=x$

$\Rightarrow \frac{20 x}{100}+40=x$

$\Rightarrow\left(\frac{20 x}{100}-x\right)=-40$

$\Rightarrow \frac{-80 x}{100}=-40$

$\Rightarrow x=\left(40 \times \frac{100}{80}\right)$

$\Rightarrow x=50$