Sometimes in word problems, you need to answer more than one question in order to get the final answer. The hidden question is the one not stated at the end of the problem. Instead, the hidden question is implied. We have been going over in class stating what is known in the problem and what is unknown. When you list this out, it will make any hidden question come to light.
Here is an example:
A farmer has 20 red apples and 15 green apples. He sold 12 apples to a friend. How many apples does the farmer have left to sell at the farmer's market?
For this problem we know the question is how many are left, but we do not know how many apples in total (both green and red) the farmer starts with. For this problem the hidden question not stated in the word problem is "how many apples does the farmer have in total." You cannot answer how many he has left without first knowing how many he had in total.