At least one jean, means that we have to find all the ways there are to select the items from 1 to 4 jeans.
If we take one Jean, there are 8 possibilities for the shirts
If we take 2 jeans, there are 16 possibilities for the shirts
if we take 3 jeans, there are 24 possibilities for the shirts
If we take 4 jeans, there are 32 possibilities for the shirts
So, 8 x 4 {32 possibilities}
