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If a light bulb is selected at random from a shipment, what

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If a light bulb is selected at random from a shipment, what [#permalink] New post 25 Oct 2009, 05:15
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If a light bulb is selected at random from a shipment, what is the probability that the light bulb is defective?


(1) The ratio of the number of defective light bulbs to the number of nondefective light bulbs is 1 to 60.

(2) The shipment contains 720 light bulbs.
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Re: prob2 [#permalink] New post 25 Oct 2009, 11:01
IMO A.

State 1) Probability = 1/61

State 2) We don't know the number of defective bulbs.
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Re: prob2 [#permalink] New post 17 Oct 2010, 09:03
IMO, the correct answer is C. lets just simply pluggin some numbers. imagine we have a total number of 100 bulbs of which 20 are defective and the rest are none-def. the ratio of def to non-def will be 20 to 80 which is equal to 1 to 4. however if we calculate the probablity based on given info, the probability of ocurring defect in bulbs will be 20 to 100 wich is equal to 1 to 5. as you see, the out come is different. we need to know the total number of light bulbs in order to calculate the probability.
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Re: prob2 [#permalink] New post 17 Oct 2010, 23:12
honeyhani wrote:
IMO, the correct answer is C. lets just simply pluggin some numbers. imagine we have a total number of 100 bulbs of which 20 are defective and the rest are none-def. the ratio of def to non-def will be 20 to 80 which is equal to 1 to 4. however if we calculate the probablity based on given info, the probability of ocurring defect in bulbs will be 20 to 100 wich is equal to 1 to 5. as you see, the out come is different. we need to know the total number of light bulbs in order to calculate the probability.


Well, in the case you proposed, imagine the only information you have is 20/80, or 1/4. Simply calculating 20+80=100 gives you the rate 20/100 or 1/5. And since probability is expressed in ratios or percentages, then 1/5 is already the solution. So you can calculate the total knowing the partial amounts.

In this exercise, with statement 1 you know that the ratio of defective to nondefective is 1/60. That means that defective to total is 1/(1+60), or 1/61. Then statement 1 is sufficient. You don't need to know the actual number of light bulbs, just the rate between defective to nondefective, or defective to total, or nondefective to total.
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Re: prob2 [#permalink] New post 20 Oct 2010, 13:25
A.

1) 1x:60X 1X/61X so 1/61 sufficient
2) insufficient
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Re: prob2 [#permalink] New post 21 Oct 2010, 07:16
can someone please post the OA ? I saw a similar question in a test and the solution said that the total number is needed to find the probability , simply knowing the ratio of one item to the other wouldnt help. Reading the posts above , I am confused . IMO the answer is C. Experts please help.
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Re: prob2 [#permalink] New post 21 Oct 2010, 07:47
+1 A for the same reasons already given
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Re: prob2 [#permalink] New post 25 Oct 2010, 10:16
it sounds like the answer is A as we're asked for the probability and not the # of defective light bulbs.
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Re: prob2 [#permalink] New post 28 Oct 2010, 01:54
it sounds like A to me
Re: prob2   [#permalink] 28 Oct 2010, 01:54
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