If #peaches = x and #plums = y we can write the following equations:
x+y=15
0.89x + 0.39y = 8.85
and x and y are whole numbers.
converting the first to x=15-y and plugging it into the second, you get
0.89(15-y) + 0.39y = 8.85 =>
13.35 - 0.89y + 0.39y = 8.85 =>
4.5 = 0.5y =>
y = 9
so x = 15-9 = 6
She bought 9 plums and 6 peaches.
