To find the percentage of students who did not wear red shirts, first determine how many students did not wear them.
Total students = 60
Students who wore red shirts = 15
Students who did not wear red shirts = Total students - Students who wore red shirts
Students who did not wear red shirts = 60 - 15 = 45
Now, to find the percentage of students who did not wear red shirts, use the following formula:
\[ \text{Percentage} = \left( \frac{\text{Number of students who did not wear red shirts}}{\text{Total number of students}} \right) \times 100 \]
\[ \text{Percentage} = \left( \frac{45}{60} \right) \times 100 \approx 75% \]
So, 75% of the students did not wear red shirts.