Well... I know that 7.8 is the closest to a tidy number, so I added 0.2 to get to 8.
Since I added 0.2 to 7.8, I have to subtract 0.2 from 3.7 which makes 3.5. We do this to make it equal and you'll get the same answer.
So I end up answering a new equation. 8+3.5 and that equals 11.5.