rahulvsd 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
[spoiler]OA:A[/spoiler]
P(exactly N times) = P(one way) * total possible ways.
One way to get a good outcome:
The first jellybean is red; the other three are green.
P(RGGG) = 1/2 * 1/2 * 1/2 * 1/2 = 1/16.
Total possible ways to get a good outcome:
Since R could be the 1st, 2nd, 3rd, or 4th jellybean selected -- implying 4 ways to get exactly one R -- the result above must be multiplied by 4.
4 * 1/16 = 1/4.
The correct answer is
A.
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