Assuming each unit pays $0.043 and there are 24 units in a box, one box pays 24×0.043=$1.032.
By proportion, $15 would pay for 15/1.032=625/43 or 14.5349 boxes approx. This is the number of boxes made in an hour. Therefore the assembly time for one box is 60/14.5349 or (43/625)60=4.128 minutes on average.