IV. 6,124 - 879 7. Here are some examples of a strategy for doing some mental arithmetic that's sometimes called 'compensation" or "trading off." Yamil most . Computing 85 - 49 is a little annoying. 85 - 50, though, is easy. 85 - 50 = 35 but I subtracted one too many so 85 - 49 = 36. (More formally, 85 - 49 - 85 - (50 -1) = (85 -50) +1) . If I want to compute 124 + 38, I can compute 124 + 40 = 164 realized that I added two too many. So 124 + 38 = 162. (More formally, 124 + 38 = 124 + (40 - 2) = (124 + 40) - 2)