represent the total number of boxes that both jean and clint stack in h hours,
total number of boxes=(50xh)+(60xh)
you want the total number of boxes for both of them together. so the amount they can stack times h or the hours. so if h=5 you would do 50x5 plus+ 60x5 which would give you 250 and 300. add the two you get 550 boxes for 5 hours total.
