A bag contains 8 red and 5 white balls. 2 balls are drawn at random. What is the probability that both are white?

n(S) = Number of ways of drawing 2 balls out of 13 = ¹³C₂ = 13 x 12 / 2= 78
n(E) = No. of ways of drawing 2 balls out of 5 = ⁵C₂ = 5 x 4 / 2 = 10
∴ P(E) = n(E)/n(S) = 10/78 = 5/39