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Sisters Audition

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by harsh.champ » Wed Feb 17, 2010 6:11 am
Two sisters A and B appeared for an audition. The probability of selection of A is 1/5 and that of B is 2/7. Find the probability that both of them are selected.



A.4/17
B.1/5
C.2/35
D.1/2
E.2/5
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by Osirus@VeritasPrep » Wed Feb 17, 2010 7:01 am
When you have an "and" probability problem you multiply both of the probabilities.

1/5 * 2/7 = 2/35. Choose C
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by money9111 » Wed Feb 17, 2010 7:59 am
Osirus - thank you for posting this "When you have an "and" probability problem you multiply both of the probabilities." because for me that'll be key to remember when to add vs. multiply.. i did get 2/35 though, but i go through this long drawn out thinking process determining when to multiply or add
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by cunazza » Wed Feb 17, 2010 8:24 am
With regard to these problems, I've also read somewhere that if intuitively the combined probability should decrease then you have to multiply otherwise you have to add.
Don't know if it's useful but just to let you guys know.
Here, the answer is clearly 2/35.
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by shashank.ism » Wed Feb 17, 2010 8:39 am
cunazza wrote:With regard to these problems, I've also read somewhere that if intuitively the combined probability should decrease then you have to multiply otherwise you have to add.
Don't know if it's useful but just to let you guys know.
Here, the answer is clearly 2/35.
Yeah and if the events are mutually exclusive... then probability of "and" is simply 0.
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by ajith » Wed Feb 17, 2010 9:10 am
harsh.champ wrote:Two sisters A and B appeared for an audition. The probability of selection of A is 1/5 and that of B is 2/7. Find the probability that both of them are selected.



A.4/17
B.1/5
C.2/35
D.1/2
E.2/5
1/5*2/7 ; Assumption, the events are independent
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by ajith » Wed Feb 17, 2010 9:13 am
shashank.ism wrote:
cunazza wrote:With regard to these problems, I've also read somewhere that if intuitively the combined probability should decrease then you have to multiply otherwise you have to add.
Don't know if it's useful but just to let you guys know.
Here, the answer is clearly 2/35.
Yeah and if the events are mutually exclusive... then probability of "and" is simply 0.
A general rule is you can add probabilities when they are mutually exclusive, If the question were what is the the probability that at least one of them would be selected?

You multiply when the events are independent (just like we did now)
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by shashank.ism » Wed Feb 17, 2010 9:30 am
ajith wrote:
A general rule is you can add probabilities when they are mutually exclusive, If the question were what is the the probability that at least one of them would be selected?

You multiply when the events are independent (just like we did now)
Yeah Ajith you have summed up the things in a very nice way. I meant to say the same. thanks..
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by money9111 » Wed Feb 17, 2010 9:39 am
@shashank.ism - your posts are very transparent, ours are as well but it's simply a defense mechanism against your offensive strategy. when will it end? i'm just curious so I can block out the appropriate time on my schedule.. thanks
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