(60(20-х)+60(20+х)) / ((20+х)(20-х)) = 25/4

(1200-60х+1200+60х) / (400-х²) = 25/4

2400 / (400-х²) = 25/4

(400-х²)*25 = 9600

10000-25х²-9600=0

400-25х²=0

25х²=400

х²=16

х=4    х=-4