Answer:
21
Step-by-step explanation:
I dunno, I used a calculator B)
Answer:
C. 
General Formulas and Concepts:
<u>Calculus</u>
- Mean Value Theorem (MVT) - If f is continuous on interval [a, b], then there is a c∈[a, b] such that

- MVT is also Average Value
Step-by-step explanation:
<u>Step 1: Define</u>

f'(c) = 20
Interval [1, b]
<u>Step 2: Check/Identify</u>
Function [1, b] is continuous.
Derivative [1, b] is continuous.
∴ There exists a c∈[1, b] such that 
<u>Step 3: Mean Value Theorem</u>
- Substitute:

- Rewrite:

And we have our final answer!
Answer = 4 ft
Radius = Circumference / (pi x 2)
Radius = 8 / (4 x 2)
Radius = 4