30% is 30/100 or 0.3. Multiply 120 by 30/100 or 0.3 and you get 36. To get rid of the decimal you can "borrow" 10 from 120 (divide 120 by 10 by simply removing the zero) to make it 12 and use the borrowed 10 to make 0.3 into 3 ( multiply 0.3 by 10). Then you just multiply 3 by 12 to get 36, because multiplying by 10 just moves the decimal point one place to the right where, in this case, it just becomes invisible since 03. is the same as 3 on its own.