Subtract 1-, 2-, 3-, 4-, or 5-digit number from a 5-digit number
In other words, they never memorized their multiplication tables! Many times I have seen a student do poorly due to a weakness in basic math facts they should have learned in third grade.
Worksheets also, when used properly, provide both the students and parent / tutor immediate feedback as to the child's progress. This means they can be used to point out areas where the student needs further reinforcement.
How much money do I have left if I buy a soda? By the end of the week, how much of my daily allowance will I be able to save if I don't?