Hi Giselle,

Since your post is titled "picking numbers", I'll use that approach:

Let's say that the store starts with 10,000 items.

Day 1: sell .20(1000) = 2000, 8000 left over

Day 2: sell .2(800) = 1600, 6400 left over

Day 3: sell .2(640) = 1280, 5120 left over

Day 4: sell .2(512) = 1024, 4096 left over

Ok, so this means out of the original 10,000, there are only 4096 left. So 10,000 - 4096 = 5904 were sold during the 4 days.

The fraction of the total = 5904/10000, which can be reduced to 369/625.

Hope that helps!