how to calculate percentage change in your head?
['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.']