If 100 students make up 40% of the 6th grade population, then 50 students make up 20% of the 6th grade population. 20% × 5 makes 100% so if your multiply 50 by 5 you'd get your population.
50 × 5 = 250 sixth-graders
We will have the following:
Using exponential regression we will have that the line of best fit will be:

Now, we will have that:

So, the approximation using linear regression gives us as solution approximately 29.7 years; thus the closest one to match is then
The radius is half the diameter. 19*2=38. The diameter is 38.