5.100 x 2%/100% <-- this is how we convert the percentage to a decimal so that we can work with it

5.1 x 0.02 <-- notice I dropped those 00's after the tenths place, we don't need them, test makers use tricks like that to make a problem seem more difficult than it really is

If you would like to do it in your head, you can move the decimal (of 5.1) two places to the left, which will give you .051 which is equivalent to 1%. Then multiply this (.051) by 2 and you will have 2% of 5.1, which is .102

Rule: Anytime you need to find 10% move the decimal to the left once. Anytime you need to find 1% move the decimal to the left two places.