WCCO had an article this morning about teaching math the Singaporan way. After reading the article, I realized that I have been doing math in my head the same way for years.
For instance if you are trying to subtract 32 from 145, I would round down to 30 and round down to 140, giving 110. Then I would subtract 2 from 5 (the leftovers from rounding) to get 3. Adding 110 and 3 gives us 113, which is the answer.