Percentages Made Simple

A percent is just a part out of 100. The word "percent" means "per hundred." So 25 percent means 25 out of every 100 – the same as a quarter of something. If a class has 100 students and 25 are absent, that's 25 percent absent.

To find a percent of a number, turn the percent into a decimal and multiply. Move the decimal point two places to the left: 20 percent becomes 0.20. So 20 percent of $50 is 0.20 times 50, which is $10. That's your discount, your tip, or your share – the same math every time.

Here is a faster trick for everyday life. To get 10 percent of any number, just move its decimal one place left: 10 percent of $80 is $8. Need 20 percent? Double the 10 percent ($16). Need 5 percent? Take half of the 10 percent ($4). You can mix these to build almost any tip in your head.

Percent change tells you how much something went up or down. Subtract the old number from the new number, divide by the old number, then multiply by 100. If rent went from $1,000 to $1,100, that's a $100 rise, divided by 1,000, which is 0.10 – a 10 percent increase. The same steps work for a price drop or a pay raise.

One thing to watch: a percent always points back to a starting amount, and that amount can change. A 50 percent discount followed by another 50 percent off is not 100 percent off. The second cut comes off the already-lower price, so you still pay something. Always ask, "Percent of what?"