You unfold chicken wire to make a circular pen. the area of the pen is 78.5 square meters. how many meters of chicken wire do you need to make the pen?

Circular pen means you are going to use Area = Pi(r^2). 

So 78.5 = pi(r^2). Divide by pi on both sides to get r^2 = 78.5/pi. Take the square root of that to get r or radius = approximately 4.99. If you need an exact answer then use  for the next part because you are not done yet.

Plug whatever R you use into the circumference formula of Circumference = 2(pi)(r).

If you use 4.99, you do 2(pi)(4.99) which is approximately 31.35. If you use the exact answer, you do 2(pi)() which gives you approximately 31.41. 

Either way you'll need about 31 meters of chicken wire but the 31.40796 or ~31.41 is closer to the exact answer.