Solution: Suppose junior high school students originally planned to donate X books, and senior high school students originally planned to donate 3500-x books.

120% x+ 1 15%(3500-x)= 4 125

1.2x+ 1. 15×3500- 1. 15x = 4 125

0.05x=4 125-4025

x=2000

3500-x=3500-2000= 1500 (volume)

Actual junior high school student donation 120%×2000=2400 (copies).

Actual high school student donation115% ×1500 =1725 (quantity).

2400-2000=400 (volume)

1725- 1500=225 (volume)

A: Junior high school students donated 400 more books than originally planned. High school students donated 225 more books than originally planned.