Suppose the number of junior high school students is X, the number of senior high school students is 5X/6, the number of junior high school graduates is Y, and the number of senior high school graduates is 12Y/ 17.

Simultaneous equation:

X-Y=520

5X/6- 12x/ 17=520

X= 1200，Y= 1000。

So there are 680 junior high school graduates, 480 senior high school graduates and * * * 1 160.