Question

An urn contains 7 red marbles and 3 white marbles. three marbles are drawn from the urn one after the other. find the probability that the first two are red and the third is white.

Answer 1

7+3 = 10 total marbles

 1st pick = 7/10 for a red one

2nd pick = 6/9 reduces to 2/3 for a red one

3rd pick = 3/8 for a white one

7/10 x 2/3 =14/30 reduces to 7/15

7/15 x 3/8 = 21/120 reduces to 7/40

 probability = 7/40