There would be 24 crates on backorder by the beginning of the fifth day. To solve this, I wrote the equation (64+[18x4])-(28x4)=b, where b is the number of crates on backorder. Also, x is the multiplication sign in the equation. I simplified the equation to be 136-112=b, so b=24. Hope that this helps! Sorry if the explanation is a bit confusing!