Problema Solution
There are 10 marbles in a bag, and the marbles are either red or blue. Eric will randomly choose two marbles from the bag, without replacing the first one. If the probability of both marbles' being red is
2
15
, how many BLUE marbles are in the bag?
Answer provided by our tutors
Let x represent the number of red marbles in the bag, x>0
10 - x is the number of blue marbles in the bag
the probability that 2 marbles are red is:
P(2 marbles are red) = x(x-1)/(9*10)
x(x-1)/(9*10) = 2/15
........
........
x = 4 red marbles
10 - x = 10 - 4 = 6 blue marbles
There are 6 blue marbles in the bag.