Answer: 40%
Explanation:
The wording 9 out of 15 can be written into a fraction.
As a fraction, this would be 9/15.
Once you simplify that, you get 3/5.
You know that there is 100% in a whole.
3 * 20 / 5 * 20 so the denominator is 100.
60/100 people got the homework correct, 60/100 can be written into 60%.
Subtract this from the total 100% and you get 40% of students who got the homework assignment wrong.