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# A certain jar contains 21 marbles, 7 of which are red. If two marbles

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Math Expert
Joined: 02 Sep 2009
Posts: 58434
A certain jar contains 21 marbles, 7 of which are red. If two marbles  [#permalink]

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18 Jan 2019, 00:50
00:00

Difficulty:

5% (low)

Question Stats:

86% (00:52) correct 14% (01:11) wrong based on 69 sessions

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A certain jar contains 21 marbles, 7 of which are red. If two marbles are to be selected, with replacement, what is the probability that both marbles will be red?

A 1/9
B 1/6
C 1/3
D 2/3
E 3/4

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Posts: 8
Re: A certain jar contains 21 marbles, 7 of which are red. If two marbles  [#permalink]

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18 Jan 2019, 04:52
A certain jar contains 21 marbles, 7 of which are red. If two marbles are to be selected, with replacement, what is the probability that both marbles will be red?

No. of ways of selecting 2 red marbles = 7* 7
No. of way of selecting any 2 marbles = 21*21
Probability of selecting 2 marbles = 7*7/21*21 = 1/9

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Re: A certain jar contains 21 marbles, 7 of which are red. If two marbles  [#permalink]

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18 Jan 2019, 07:19
Bunuel wrote:
A certain jar contains 21 marbles, 7 of which are red. If two marbles are to be selected, with replacement, what is the probability that both marbles will be red?

A 1/9
B 1/6
C 1/3
D 2/3
E 3/4

7/21 * 7/21 = 1/9
IMO A
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Re: A certain jar contains 21 marbles, 7 of which are red. If two marbles  [#permalink]

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26 Feb 2019, 10:34
Can anyone help me crack this question with help of Combination approach (nCr) ?
i did:
7c1*7c1/21c2
so i got
49/210= 7/30.
What am i doing wrong ?
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Re: A certain jar contains 21 marbles, 7 of which are red. If two marbles  [#permalink]

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01 Mar 2019, 10:49
Bunuel wrote:
A certain jar contains 21 marbles, 7 of which are red. If two marbles are to be selected, with replacement, what is the probability that both marbles will be red?

A 1/9
B 1/6
C 1/3
D 2/3
E 3/4

The probability of selecting 2 red marbles is 7/21 x 7/21 = 1/3 x 1/3 = 1/9.

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Re: A certain jar contains 21 marbles, 7 of which are red. If two marbles  [#permalink]

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21 Mar 2019, 05:29
hsn81960 wrote:
Can anyone help me crack this question with help of Combination approach (nCr) ?
i did:
7c1*7c1/21c2
so i got
49/210= 7/30.
What am i doing wrong ?

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Re: A certain jar contains 21 marbles, 7 of which are red. If two marbles   [#permalink] 21 Mar 2019, 05:29
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