JAMB 2003 · UME · Q16

Find the values of $x$ and $y$ respectively if $3x - 5y + 5 = 0$ and $4x - 7y + 8 = 0$ .

Worked solution (try it first)

Move the numbers across with their signs changed: $3x - 5y = -5$ and $4x - 7y = -8$ .
Make the $x$ terms match: multiply the first by 4 and the second by 3, giving $12x - 20y = -20$ and $12x - 21y = -24$ .
Subtract the second from the first: $y = 4$ .
Then $3x = 5y - 5 = 15$ , so $x = 5$ .
So $x, y$ are $5, 4$ , option C.