For your problem, you would have to covert 10% into a decimal = .10 by moving the decimal point 2 times to the left since percent is based from 100. Then you would multiply .10 by 30 and get 3.
In a simpler way, you can move the decimal point once to the left since 10% has one zero. If you do that you get 3. That is just a small, efficient trick.
Hope you find this useful :)
