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. How many BLUE marbles are in the bag given that the probability of both marbles will be red is 2 15?

Question

Geography

Giovanna Mcbride

30 September, 10:00

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. How many BLUE marbles are in the bag given that the probability of both marbles will be red is 2 15?

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Answers (1)

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Hugo Jones · Answer 1

6 blue marbles

Explanation:

(red / 10) * (red minus one / 9)

r (r-1) / 90

2/15 = 12/90

r (r-1) / 90 = 12/90

r² - r = 12

r² - r - 12 = 0

(r + 3) (r - 4) = 0

r = - 3 or r = 4

red = 4; blue = 6