This video explains how to calculate percentages in your head. To calculate a percentage in your head, start by breaking the percentage off into smaller units. For example 25% is 10 + 10 + 5. 30% is 10, 10, and 10. Now you have numbers you can easily work with in your head. Find 10% of the number. This is easily done by one of two methods. You can either move the decimal point one place to the left or you can drop the last number. Dropping the last number sometimes gives you an estimate, though, if that number is not zero. For example, if you are trying to find 25% off of a $40.00 item, then your could move the decimal point in 40 to the left to get 4.00. You could also drop the 0 on the end and get 4. Four dollars is ten percent of 40. Now add the answer up to get the original percentage. For example, if you were calculating 30 percent off of $40.00, then you broke the 30 into 10,10, and 10. Ten percent of 40 is 4. Now you need to add that three times to get the 30. Your discount is 12.00 If you were figuring 35% off then you would add it three times and add half as much again (or add half of 4, or 2, to the 12 to get 14).