The easiest way to solve for this is to first find out how many he has sold right now. The way to find it is to divide the total amount of money (301) by the amount of each cup. (301/3.50) That should be equal to 86 cups. Now, you want to find the rest he needs to sell. The easiest way is to divide the number you want (371) by the number of each cup (371/3.50) That should be equal to 106. Since you want to know how many more he needs to sell you now need to subtract them. (106-86) That is equal to your answer, 20.
