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In a box with 10 blocks, 3 of which are red, what is the probability o

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Bunuel
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In a box with 10 blocks, 3 of which are red, what is the probability o [#permalink] New post   02 Jul 2019, 04:27
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In a box with 10 blocks, 3 of which are red, what is the probability of picking out a red block on each of your first two tries? Assume that you do NOT replace the first block after you have picked it.

A. 1/15
B. 9/100
C. 2/15
D. 9/50
E. 9/25
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A
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Re: In a box with 10 blocks, 3 of which are red, what is the probability o [#permalink] New post   02 Jul 2019, 04:37
Bunuel wrote:
In a box with 10 blocks, 3 of which are red, what is the probability of picking out a red block on each of your first two tries? Assume that you do NOT replace the first block after you have picked it.

A. 1/15
B. 9/100
C. 2/15
D. 9/50
E. 9/25


Probability = 3/10 * 2/9 = 1/15

IMO A
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Lampard42
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Re: In a box with 10 blocks, 3 of which are red, what is the probability o [#permalink] New post   02 Jul 2019, 05:36
Bunuel wrote:
In a box with 10 blocks, 3 of which are red, what is the probability of picking out a red block on each of your first two tries? Assume that you do NOT replace the first block after you have picked it.

A. 1/15
B. 9/100
C. 2/15
D. 9/50
E. 9/25


each of your first two tries and NOT replace So prob of 1st*2nd = 3/10*2/9 = 1/15 IMO A
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Re: In a box with 10 blocks, 3 of which are red, what is the probability o [#permalink] New post   02 Jul 2019, 09:48
Bunuel wrote:
In a box with 10 blocks, 3 of which are red, what is the probability of picking out a red block on each of your first two tries? Assume that you do NOT replace the first block after you have picked it.

A. 1/15
B. 9/100
C. 2/15
D. 9/50
E. 9/25


first two tries picking red block ; 3/10*2/9 ;1/15
IMO A
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Re: In a box with 10 blocks, 3 of which are red, what is the probability o [#permalink] New post   02 Jul 2019, 12:50
Bunuel wrote:
In a box with 10 blocks, 3 of which are red, what is the probability of picking out a red block on each of your first two tries? Assume that you do NOT replace the first block after you have picked it.

A. 1/15
B. 9/100
C. 2/15
D. 9/50
E. 9/25


Without replacement
Total = 10 (3 reds, 7 non reds)
Prob(red) = 3/10
Total = 9 (2 reds, 7 non reds)
Prob(red) = 2/9

Required Probability = 3/10*2/9 = 6/90 = 1/15

IMO Option A

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In a box with 10 blocks, 3 of which are red, what is the probability o [#permalink] New post   03 Jul 2020, 19:19
Bunuel wrote:
In a box with 10 blocks, 3 of which are red, what is the probability of picking out a red block on each of your first two tries? Assume that you do NOT replace the first block after you have picked it.

A. 1/15
B. 9/100
C. 2/15
D. 9/50
E. 9/25



I understand the answer (3/10x2/9) but i have a question that confuse me sometimes. Why don't we consider the other possibility [(3/10 x 2/9)+ (2/10 x 3/9)]= 2/15
IMO As we are dealing with just one color so there cant be different combinations :)
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Re: In a box with 10 blocks, 3 of which are red, what is the probability o [#permalink] New post   29 Nov 2022, 06:18
Why doesn't the 1 - (not red) work? If we do:
1st pick not red= 7/10
2nd pick not red = 6/9

Probability of picking red on both tries = 1 - (7/10)*(6/9) = 8/15

Why does it give a different answer?
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Re: In a box with 10 blocks, 3 of which are red, what is the probability o [#permalink] New post   20 Dec 2022, 08:40
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rol6 wrote:
Why doesn't the 1 - (not red) work? If we do:
1st pick not red= 7/10
2nd pick not red = 6/9

Probability of picking red on both tries = 1 - (7/10)*(6/9) = 8/15

Why does it give a different answer?


Because you forgot to calculate the probability of drawing only one red ball.
You were calculating the probability of drawing no red balls when you need to calculate the probability of drawing not 2 red balls. Which would be the following:
(7/10)*(6/9)+(3/10)*(7/9)+(7/10)*(3/9)=14/15
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Re: In a box with 10 blocks, 3 of which are red, what is the probability o [#permalink]
20 Dec 2022, 08:40
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