let x = # of fittings checked per day
960 = 15x
Solve for x:
1. Divide by 15
64 = x
This tells us the plumber can check 64 fittings per day. This is not the answer quite yet. Since he only works 1/4 a day, multiply this 64 by 1/4:
64 * 1/4 = 16
The answer is 16 fittings.