Hello there!
Your correct answer will be 35 people under 16.
You know that 1/3 of the people are teachers, which means that 2/3 of people are left. Now, you need to get to the decimal value (.667). Make that number 1.667, because there will not be less teachers than students. Now, multiply:
1.667 x 21 = 35
There you go!
Brady