Free interactive math game: Using Known Facts — If 7+8=15, then 15-8=?. Part of the Addition & Subtraction category on MathKite.
Taken from the generator that powers this exercise. A fresh set appears every time the page is opened, and the printable worksheet holds ten of them.
Using one fact you already know to reach a related one without recalculating.
Notice how the new question differs from the fact you are given, and apply that same change to the answer.
If 9 x 9 = 81, then 90 x 9 is ten times bigger: 810.
Recomputing from scratch and losing the connection. The point is to see the relationship, which is what makes mental arithmetic fast.