Probability (P) (k successes out of n trials) = nCk * p^k * (1-p)^(n-k), where nCk = n! / (k! * (n-k)!)
for this problem n=20, p = 1/6(probability of rolling 1 two) is approximately 0.1667
P ( of getting more that 3 two's) = 1 -P(0 two's out of 20 trials) -P(1 two out of 20 trials) -P(2 two's out of 20 trials) -P(3 two's out of 20 trials)
P ( of getting more that 3 two's) = 1 -0.026 -0.104 -0.198 -0.238 = 0.434
