Mr. Lake's Math Site

Percents %

Before you can do anything mathy with a percent, you have to change it into a decimal.  This is really easy, just move the decimal two spots to the left.  This means that 5%=0.05, 0.08%=0.0008, and 20%=.2.

To take a percent of something, turn it into a decimal by moving the decimal point over two spots to the left, then just multiply.  If we wanted to know 5% of 378, then we just multiply 0.05*378 and we get our answer of 18.9.

Sometimes when you are using percents, you are not able to use a calculator, it is easier to think of them as a ratio.  Ten percent means ten for each hundred.  That means if we wanted 10% of 400, we have four tens, or 40.  

There is only 100%.  When your coach tells you to give 110%, they are clearly showing that they should not be teaching a math class.  There is no 110%.  This is important in percent problems because all of our stuff needs to add up to 100%.  If we know that 25% is one thing, the rest has to be 75%.

Sometimes we need to undo a percent.  If there was a sale and they want to know what the price was before the discount, we just divide the sales price by the decimal version of the percent.  This means if the sales price is $45 and the shirt is 30% off, to find the original price, we just divide the $45 by 0.7 to get the original amount.