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

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Director
Joined: 01 Apr 2008
Posts: 881
Name: Ronak Amin
Schools: IIM Lucknow (IPMX) - Class of 2014
If a light bulb is selected at random from a shipment, what [#permalink]

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25 Oct 2009, 05:15
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71% (01:31) correct 29% (00:31) wrong based on 103 sessions

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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.
[Reveal] Spoiler: OA

Last edited by Bunuel on 27 May 2016, 07:59, edited 2 times in total.
Senior Manager
Joined: 18 Aug 2009
Posts: 320
Re: If a light bulb is selected at random from a shipment, what [#permalink]

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25 Oct 2009, 11:01
IMO A.

State 1) Probability = 1/61

State 2) We don't know the number of defective bulbs.
Intern
Joined: 16 Oct 2010
Posts: 7
Re: If a light bulb is selected at random from a shipment, what [#permalink]

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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.
Joined: 19 Feb 2010
Posts: 394
Re: If a light bulb is selected at random from a shipment, what [#permalink]

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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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Joined: 15 Jul 2010
Posts: 250
GMAT 1: 750 Q49 V42
Re: If a light bulb is selected at random from a shipment, what [#permalink]

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20 Oct 2010, 13:25
A.

1) 1x:60X 1X/61X so 1/61 sufficient
2) insufficient
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Intern
Joined: 28 Sep 2010
Posts: 5
Re: If a light bulb is selected at random from a shipment, what [#permalink]

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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.
Manager
Joined: 06 Aug 2010
Posts: 220
Location: Boston
Re: If a light bulb is selected at random from a shipment, what [#permalink]

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21 Oct 2010, 07:47
+1 A for the same reasons already given
Manager
Joined: 24 Jul 2010
Posts: 89
Re: If a light bulb is selected at random from a shipment, what [#permalink]

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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.
Manager
Joined: 24 Jul 2010
Posts: 89
Re: If a light bulb is selected at random from a shipment, what [#permalink]

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28 Oct 2010, 01:54
it sounds like A to me
Intern
Joined: 03 Apr 2016
Posts: 20
Re: If a light bulb is selected at random from a shipment, what [#permalink]

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27 May 2016, 07:26
1
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Math Expert
Joined: 02 Sep 2009
Posts: 39673
Re: If a light bulb is selected at random from a shipment, what [#permalink]

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27 May 2016, 08:00
pvikas wrote:

________________
Done. Thank you.
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Director
Joined: 04 Jun 2016
Posts: 650
GMAT 1: 750 Q49 V43
If a light bulb is selected at random from a shipment, what [#permalink]

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04 Aug 2016, 05:43
Economist wrote:
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.

(1) The ratio of the number of defective light bulbs to the number of non-defective light bulbs is 1 to 60.
Defective bulb = x
Non defective bulbs = 60x
Total Bulbs = x+60x=61x
Probablity (Defective Bulb)= $$\frac{Defective}{Total} = \frac{x}{61x} = \frac{1}{61}$$
SUFFICIENT

(2) The shipment contains 720 light bulbs.
We are not given the any info about defective bulbs or good bulbs or any other info in ratio or proportions
INSUFFICIENT

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If a light bulb is selected at random from a shipment, what   [#permalink] 04 Aug 2016, 05:43
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