WAEC 2019 · Paper 1 · Q16

If $3p = 4q$ and $9p = 8q - 12$ , find the value of $pq$ .

Worked solution (try it first)

From $3p = 4q$ , divide by 4: $q = \frac{3p}{4}$ , so $8q = 6p$ .
Substitute into the second equation: $9p = 6p - 12$ , so $3p = -12$ and $p = -4$ .
Then $q = \frac{3 \times (-4)}{4} = -3$ .
So $pq = (-4)(-3) = 12$ , option A.