Question: how to calculate percentage change mentally?
Answer: 90% of a quantity. First find 10% of the quantity and then subtract that 10% from 100% of it. 25% of a quantity. 25% is the same as 1/4. So to find 25% of a quantity, divide it by 4. 12% of a quantity. First find 10% of it. ...  75% of a quantity. 75% is 3/4.