The students sell 9 cupcakes every 2 hours, that means that theoritycally they sell 9/2=4.5 cakes per hour
In 5 hours they will sell 18 pieces for the first 4 hours. for the rest 1 hour they will sell 4-5 pieces more
so fir 5 hours they will sell 18+4=22 or 18+5=23 pieces this is because they can not sell one half of the cake.