Let see If I get this one right.
I doesn't matter how many orders are there but it matters how many days it takes each one to complete. If they both work together then equation I believe is
(1/T1) + (1/T2) = (1/T)
so 1/4 + 1/5 = 1/T, that simplifies to 20/9 which is about 2.2. what are the other choices. were they like 2, 3, 4, 5, like that or with decimals like 2.5, 2.7, 3. I think becasue it takes more than two days so if you have answers in whole days then 3 would be right answer becasue it won't be complete in two days. You are right. It's total BS. If anyone else knows another way of doing it please post.