Question: how to calculate percentage change in your 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.