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A jar contains 16 marbles, of which 4 are red, 3 are blue, a

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A jar contains 16 marbles, of which 4 are red, 3 are blue, a [#permalink] New post 14 Feb 2011, 16:24
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A jar contains 16 marbles, of which 4 are red, 3 are blue, and the rest are yellow. If 2 marbles are to be selected at random from the jar, one at a time without being replaced, what is the probability that the first marble selected will be red and the second marble selected will be blue?

A. 3/64
B. 1/20
C. 1/16
D. 1/12
E. 1/8
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Re: A jar contains 16 marbles, of which 4 are red, 3 are blue, a [#permalink] New post 14 Feb 2011, 16:47
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banksy wrote:
A jar contains 16 marbles, of which 4 are red, 3 are blue, and the rest are yellow. If 2 marbles are to be selected at random from the jar, one at a time without being replaced, what is the probability that the first marble selected will be red and the second marble selected will be blue?

A. 3/64
B. 1/20
C. 1/16
D. 1/12
E. 1/8


Note that we want the first marble selected to be red and the second marble selected to be blue.

P=4/16*3/15=1/20.

Answer: B.
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Re: A jar contains 16 marbles, of which 4 are red, 3 are blue, a [#permalink] New post 17 Feb 2011, 10:19
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banksy wrote:
A jar contains 16 marbles, of which 4 are red, 3 are blue, and the rest are yellow. If 2 marbles are to be selected at random from the jar, one at a time without being replaced, what is the probability that the first marble selected will be red and the second marble selected will be blue?

A. 3/64
B. 1/20
C. 1/16
D. 1/12
E. 1/8


Its an "and" question so think multiplication.

Let P{a}= P{1st marble is red}= \frac{4}{16}
P{B}=P{2nd marble is blue}=\frac{3}{15}

P{A}andp{B}=\frac{4}{16}*\frac{3}{15}= \frac{1}{20}

Answer B.
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Re: A jar contains 16 marbles, of which 4 are red, 3 are blue, a [#permalink] New post 17 Feb 2011, 11:11
gmatpapa, if we have "and" so it is multiplication, and if "or" it means addition?
Senior Manager
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Status: Up again.
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Re: A jar contains 16 marbles, of which 4 are red, 3 are blue, a [#permalink] New post 17 Feb 2011, 11:39
banksy wrote:
gmatpapa, if we have "and" so it is multiplication, and if "or" it means addition?


When you are determining the probability of one event AND another event, multiply the two probabilities together. Add the probabilities if you are determining the probability of one event OR another another event.

Example. The probabilities of chip and dale scoring 700 on GMAT are 1/10 and 1/15 respectively. What is the probability that:

1. Both Chip AND Dale score a 700: Since this is an AND question, you multiply them together. Therefore,
P{both score 700}= \frac{1}{10}*\frac{1}{15}=\frac{1}{150}

2. Either Chip OR Dale scores a 700: P{Chip scores 700}+P{Dale scores 700}-P{both score 700}= \frac{1}{10}+\frac{1}{15}-\frac{1}{150}= \frac{12}{75}.
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Re: A jar contains 16 marbles, of which 4 are red, 3 are blue, a [#permalink] New post 17 Feb 2011, 13:54
nice example=) thanks a lot)
Re: A jar contains 16 marbles, of which 4 are red, 3 are blue, a   [#permalink] 17 Feb 2011, 13:54
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