Work it backwards.... you know you have 47 to begin with. One the third and last week, half were sold, so multiply by two.then add 56 to represent the second week. At this point, you should have 150. On the first week, a third were sold, so I divided by 3 and got 50 and added it to 150 to get 200. You could then check and see if you get 47 as a total.
