Can someone explain to me how to answer this question and also the equation to find the answer

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.

Gotta be d the music of previous eras