To solve the exercise you must take into account that the total number (80 problems) must be taken as 100% of the exercises, with which, you can identify the number of problems that corresponds to 20% using a rule of three, as shown then:
80 problems = 100%
X = 20%
Where X is the number of exercises that corresponds to 20%, then you proceed to operate:
<u>X = (20%*80)/100% = 16</u>
So 20% of 80 problems, which was the number Harry hit, is 16 problems.
