Part 2: 8 feet diameter = Pi * 4 squared. 8 feet is the diameter so the radius is 4. 4 squared is 16. 16 * pi = approximately 50.27, but is 50.24 if 3.14 is used as pi.
Part 3: Pi * 5 squared since 10 is diameter. 25 * pi which is close to 78.54, but is 78.5 is 3.14 is used for pi.