Question

add or subtract mentally 16.9+ 12.1

Answer 1

When you add it mentally you'd get 29
When you subtract it mentally you'd get 4.8

Answer 2

Well, just add or subtract it form the back and work your way to the front, and you have to remember what the digit was.

16.9 + 12.1 = 29
16.9  - 12.1 = 4.8