Billy sold 100 apples in January, sales went up by 10% in the second week, 20% in the third week, and went down by 50% in the fourth and final week.

Just a thought I'm not 100% on this

100 apples = total sold

Then rearrange and solve for x.

Unless, the growth is based off of the # sold in each week (i.e. sold 20 apples in week 1 then 22 in week 2 and then 26 and then 13 the last week...) So assuming that X is based off the first weeks sales, I think this should work