That's basic arithmetic.
20.00-15.24=?
Liz wants to buy her favorite musical group’s new CD. The CD costs $15.24, including tax. Liz gives the store clerk a twenty-dollar bill. How much change should Liz get back?
With as simple a math question as this is, I hope you're under 10 years old, but on that if you are should not me on this site.