You can find worksheets for a wide range of courses--almost any course you want to teach your children. These include spelling, writing, English, history, math, music, geography, and others.
They're also available for nearly all grade levels. There are printable middle school, high school, elementary school, and even pre-school worksheets.
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?
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.