A bag contains 3 black balls,4yellow balls . find probability that both are of the same colour if 2balls are picked at random with out replacement.

They must either be both black or both yellow

prob(of that event) = (3/7)(2/6) + (4/7)(3/6) = 3/7

or

prob = ( C(3,2) + C(4,2) )/C(7,2) = (3 + 6)/21 = 9/21 = 3/7