WAEC 2020 · Paper 1 · Q4

Find the least value of $x$ which satisfies the equation $4x \equiv 7 \pmod 9$ .

Worked solution (try it first)

$4x$ must leave remainder 7 when divided by 9, so list the numbers $7, 16, 25, 34, \ldots$ (add 9 each time).
The first that is a multiple of 4 is 16, so $4x = 16$ and $x = 4$ .
Check: $4 \times 4 = 16 = 9 + 7$ .
So $x = 4$ , option D.