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Prob that Sally will not win = 4/5 Prob that Jeny will not win = 4/5

Hence 4/5 * 4/5 = 16/25

correct?

Can someone explain the flaw here?

5 women are in a race. What is the probability that Sally and Jenny both will not win the race?

Say S,J, X1,X2,X5 are in the race.. [ assuming the only one person will win the race out of 5 with equal probablity] All these events[Winning S, Winning J, Winning X1.] are "mutually exclusive" --> Out comes are not common. P(S) = 1/5 P(J) =1/5 P(X1)=1/5 P(X2)=1/5 P(X3)=1/5

probability of either one of them winning hte race = P(S or J or X1 or X2 or X3) =P(S) +P(J)+P(X1)+P(X2)+P(X3) =1 probability that Sally and Jenny both will not win the race = probability that either one of X1 or X2 or X2 them win the race = P(X1)+P(X2)+P(X3) =1/5+1/5+1/5=3/5

What is the probability that Sally and Jenny both will not win the race?

What is the probability that Sally, but not Jenny, will win the race?

Guess the question wording is not correct logically...

2 people can not win a race together... hence instead of saying..."probability that Sally and Jenny both will not win the race".. it should be "probability that Sally or Jenny both will not win the race"

Even with the second question - it should be - "What is the probability that Sally, will win the race?"... we don't need to specify not Jenny.. as Sally and Jenny cannot win the race at the same time
_________________

Cheers! JT........... If u like my post..... payback in Kudos!!

|Do not post questions with OA|Please underline your SC questions while posting|Try posting the explanation along with your answer choice| |For CR refer Powerscore CR Bible|For SC refer Manhattan SC Guide|

~~Better Burn Out... Than Fade Away~~

gmatclubot

Re: combinatorics - Sally and Jenny
[#permalink]
16 Feb 2010, 12:41

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