Idon't understand how to multiply decimals. can someone ?

77.2 x 0.272

The answer is 20.9984.

In order to solve a multiplication problem with decimals you would first multiply as if it weren't with decimals.
Once you finish adding the 3 rows of numbers (multiplying without decimals) you would then count how many numbers are behind the decimal point. In this problem there are 4 numbers behind the decimal. (Count how many are behind both)
Starting at the first number written down on the right count 4 spaces between numbers. When you reach the 4th place the decimal there.

Set square i think so