Question

6.56+ =6.6
What is the answer ??

Answer 1

If you mean 6.56 + 6.6, the answer is 13.16.

Answer 2

You will need to do the reverse operation to get the answer quickest. 
So 6.6 - 6.56 = ?.  Think of it as 60 - 56. I think you know it is four. 
So 6.6 - 6.56 = .04   Now the reverse is 6.56 + .04 = 6.6