GRE Sample Test 3 Quantitative Reasoning 1

Question : 20 of 20

Marks: +1, -0

A certain jar contains 5 marbles, r of which are red. If two marbles are to be selected at random, and the probability that both marbles will be red is

1/{10}

, what is the value of r?

Validate

Solution:

(probability of choosing r) × (probability of choosing r again) =

1/{10}

→

r/5 {r-1}/{4}

1/{10}

→

{r(r-1)}/{20}

1/{10}

→ r (r - 1) = 2 → r = 2

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