Question
An urn contains 10 black and 5 white balls. Two balls are drawn from the urn one after the other without replacement. What is the probability that both drawn balls are black?
Solution
Number of black balls = 10
Number of white balls = 5
∴ total number of balls = 10 + 5 = 15
Let E and F denote respectively the events that first and second ball drawn are black.
Now P(E) = P(black ball in first draw) = 
Since there is no replacement
By multiplication rule of probability,
