Machine A produces nails at a uniform rate of 60 every 20 seconds and machine B produces them at 50 every 10 seconds. If the two machines run simultaneously, how many seconds would it take to produce a total of 400 nails?
Rate of A = 60 nails / 20 sec = 3 nails/sec
rate of B = 50 nails / 10 sec = 5 nails/sec
combined rate = 8 nails/sec
time at combined rate = 400/8 seconds
= 50 seconds