She bought N pencils last month for 288, as the amount is in pennies. This month she bought N+12 for 720.
288/N = 720 / (N+12)
288(N+12) = 720N
288N + 3456 = 720N
3456 = 720N - 288N
3456=432N
N=8
She bought 8 pencils last month for 288, which is 36 cents each. And she bought 20 pencils this month for 720 which is 36 cents each. Hope this helped :)
