With the possible exception of 50%, there’s no percentage easier to calculate than 10. Just move the decimal place one to the left. 100 becomes 10, 55 becomes 5.5, and 0.8 becomes 0.08.
Of course, the GMAT usually doesn’t ask you to come up with ten percent of things. However, ten percent (and, by extension, one percent) is the fundamental building block of all other percents. Let’s start with a couple of simple examples.
Just like 20% is easy (once you have 10%), 5% and 1% are simple, as well. If you know that 10% = $6.50, you’re one small step away from knowing that 5% = $3.25 and 1% = $0.65. So, if you wanted to calculate 16%, you could simply add the three.