Amount in compound interest = p(1 + r/t)^nt where p is the initial deposit, r = rate, t = number of compunding in a period and n = period.
Here, Amount after 20 years = 100(1 + (6/100)/12)^(20 x 12) = 100(1 + 0.06/12)^240 = 100(1 + 0.005)^240 = 100(1.005)^240 =100(3.31020) = $331.02
