If three marbles are picked at random, what is the probability that either all are green or all are red?

Total number of ways of selection of 3 marbles out of 12
= n(S) = ¹²C₃ = 220
Total number of favorable events = n(E)
= ³C₃ + ⁴C₃ = 1 + 4 = 5
∴ Required probability
= 5/220 = 1/44