The number 18 is a 120% of the number 15. That is because when you divide the two numbers you get 1.2. Then you multiply this number by a hundred to see what the percentage number is and you get 120 which means that 18 is a 120% of the number 15. Similarly, if you had for example 7, you would divide 7 by 15 and multiply and see that it's just under 50%. This is the basic formula for calculating percentage.
