60x + 25y = 355 / *(-2)
120x + 30y = 570
_______________
-120x - 50y = -710
120x + 30y = 570
_______________
-20y = -140
y = 7
x = (570 - 30*7)/120 = 360/120 = 3
The cost of one pack of ballpoint pens is 3$ and the cost of one pack of gel pens is 7$.