Free interactive math game: Mental Math: +9, +11, +19 — Trick: ±10, then adjust. 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.
Adding or subtracting 9, 11 or 19 in your head by leaning on the nearby round number.
Use the round ten and then correct. Nine is ten less one, eleven is ten plus one, and nineteen is twenty less one. Subtraction works the same way in reverse.
For 65 + 19, add 20 to get 85, then take one back: 84. For 62 - 9, take 10 to get 52, then give one back: 53.
Correcting in the wrong direction. After adding 20 for 19 you subtract one, but after subtracting 10 for 9 you add one.