Johnathan plans on using 2" x 12" boards that cost $2.50 per foot. Assuming he can purchase the exact amount he needs, how much will it cost to purchase the wood?

A= pi * r^2
800 = 3.14 * r^2
800/3.14 = r^2
255 = r^2
sqrt 255 = r
15.9 rounds to 16mm = r

check..
a = 3.14 * 16^2
a = 3.14 * 256
a = 803. in between 800 and 900

so ur answer, i believe, is 16mm because 17mm would be too much and 15mm wouldn't be enough