Tell me about a time an employee made a significant mistake. What action did you take?

Ten years ago I received a complaint that the carton’s supplied (ten products) contain a larger number of products.The owner said we losses much money because we give each customer one product as a gift for ten products I studied the problem and found that one of the workers (High school) do not know simple addition and subtraction up to  tenI transfer him to another  section does not depend on the use of numbers and doing a technical assistant