divyanshig wrote:
Two identical urns—black and white—each contain 5 blue, 5 red and 10 green balls. Every ball
selected from the black urn is immediately returned to the urn, while each ball selected from the white urn is
removed and placed on a table. If Jenny receives a quarter for every blue ball, a dime for every red ball and a
nickel for every green ball she selects, what is the probability that she will be able to buy a 25-cent candy bar
with the proceeds from drawing four balls—two from each urn?
A) 143/152
B) 143/76
C) 121/120
D) 271/965
E) 152/1000
Firstly the Probability can never be GREATER than 1, probability of 1 itself means that the event is sure to happen..
So choices B and C are flawed....
Of course the choices should be in some order...
Back to the question...
So we are looking at price equal to or more than 25 cents...
Only way it will be less than 25 will be when we get a nickel in each draw, so let us choose easier path with lesser calculations....
So first draw from black a will be 10/20 and the second too will be 10/20..
First draw in white urn will be 10/20 but the second will be 9/19 ..
Probability of not being able to buy... (10/20)(10/20)(10/20)(9/19)=(1/2)(1/2)(1/2)(9/19)=9/(2*2*2*19)=9/152..
So probability of buying is 1-(9/152)=143/152
A
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