Answer: (7, 5)
Work Shown:
x = number of 30 second ads
y = number of 60 second ads
x+y = 12
y = 12-x
20x+35y = 315
20x+35(12-x) = 315
20x+420-35x = 315
-15x+420 = 315
-15x = 315-420
-15x = -105
x = -105/(-15)
x = 7
y = 12-x
y = 12-7
y = 5
The solution is (x,y) = (7, 5)