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probability

by rahul.s » Sun Jan 31, 2010 3:33 am
A jar contains equal amounts of green jellybeans and red jellybeans. Mark selects at random one jellybean at a time from the jar, notes its color, and returns it to the jar. If he repeats this process four times, what is the probability that he selects 3 green jellybeans and 1 red jellybean?

A) 1/4
B) 3/16
C) 1/8
D) 1/16
E) 1/64

OA: A
Last edited by rahul.s on Sun Jan 31, 2010 5:01 am, edited 1 time in total.
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by ajith » Sun Jan 31, 2010 3:44 am
rahul.s wrote:A jar contains equal amounts of green jellybeans and red jellybeans. Mark selects at random one jellybean at a time from the jar, notes its color, and returns it to the jar. If he repeats this process four times, what is the probability that he selects 3 green jellybeans and 1 red jellybean?

A) 1/4
B) 3/16
C) 1/8
D) 1/16
E) 1/64

OA: A

[spoiler]my solution: i assumed 4 jellybeans in each jar. we need 3 green and 1 red = (1/2 + 1/2 +1/2) * 1/2 = 3/6 * 1/2 = 1/4. but the OE was different. is it correct?[/spoiler]
Total no of outcomes = 2^4

Favorable outcomes = 4 ( RRRG, GRRR,RRGR, RGRR)

Probability = 4/16 =1/4
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by rahul.s » Sun Jan 31, 2010 3:54 am
ajith,

could you please simplify that for me? why is it 2^4? why are the the favorable outcomes = 4? where did i err?
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by ajith » Sun Jan 31, 2010 3:58 am
rahul.s wrote:ajith,

could you please simplify that for me? why is it 2^4? why are the the favorable outcomes = 4? where did i err?
From The bag you can either take out a Red one or a green one Denoted By R or G

Now u can make the first choice in 2 ways, second in 2 , third in 2 and the fourth in 2 (either R or G)

hence total events = 16

Now favourable events are ( RRRG, GRRR,RRGR, RGRR) out of all these
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by bhumika.k.shah » Sun Jan 31, 2010 5:01 am
Rahul how is 1/2 + 1/2 + 1/2 = 3/6??
shouldnt it be 3/2??
rahul.s wrote:A jar contains equal amounts of green jellybeans and red jellybeans. Mark selects at random one jellybean at a time from the jar, notes its color, and returns it to the jar. If he repeats this process four times, what is the probability that he selects 3 green jellybeans and 1 red jellybean?

A) 1/4
B) 3/16
C) 1/8
D) 1/16
E) 1/64

OA: A

[spoiler]my solution: i assumed 4 jellybeans in each jar. we need 3 green and 1 red = (1/2 + 1/2 +1/2) * 1/2 = 3/6 * 1/2 = 1/4. but the OE was different. is it correct?[/spoiler]
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by bhumika.k.shah » Sun Jan 31, 2010 5:16 am
Hey ,
can anyone explain the formula way ?? if there are 4 green colored jellybeans and 4 red colored jelly beans????
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by viidyasagar » Wed Sep 15, 2010 6:17 pm
bhumika.k.shah wrote:Hey ,
can anyone explain the formula way ?? if there are 4 green colored jellybeans and 4 red colored jelly beans????
I used Stuart Kovinsky's coin flip approach to solve this question in less than 30 seconds!!!

the real challenge, though, is to figure that this question indeed merits a coin flip solution


https://www.beatthegmat.com/coin-flip-qu ... html#75414

The link also contains a formula based approach!!!
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