That's curious. I initially tried a solution like this and got a result that differed from the expected result by a small amount. I discarded this solution and looked for others when rajatsuri said the first part of q2 assumes that the customer has already picked items that appear to be available.

Edit: By turning the 2 assumptions you mentioned on or off, I am only able to produce answers that are slightly too large or slightly too small. I wonder what the correct question is...