Question: how to calculate percentages in head?
Answer: To find 5%, find 10% and divide it in two. To find 15%, find 10%, then add 5%. To find 20%, find 10% and double it. To find 25%, find 50% and then halve it. To find 60%, find 50% and add 10%. To find 75%, find 50% and add 25%. ... and so on.