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Re: Probability clarification [#permalink]
23 Jul 2010, 08:15

rxs005, First problem is 5C2 = 10 .. Not 20..

Point 2, you sum should be greater than 4, so these are the following possibilities you have ({1,4},{1,5},{2,3},{2,4},{2,5},{3,4},{3,5},{4,5}) which leads to 8 pairs

Re: Probability clarification [#permalink]
23 Jul 2010, 12:06

Expert's post

rxs0005 wrote:

If two different numbers are randomly selected from set { 1, 2, 3, 4, 5} what is the probability that the sum of the two numbers is greater than 4?

OA ia 4/5

My approach was

total outcomes 5 C 2 = 20

outcomes where sum > 4 are

1,4 1,5 = 2 2,3 2,4 2,5 = 3 3,2, 3,3 3,4 3,5 = 4

4 any of the 5 = 5 5 any of the 5 = 5

total 19

so P(>4) = 19/20

why is this wrong

Another approach: \(P(of \ event \ X)=1-P(of \ opposite \ event \ X)\). Opposite event would be if we choose 2 different number so that their sum will be less or equal to 4.

# of total outcomes is \(C^2_5=10\) (total # of choosing 2 different numbers from the set of 5 different numbers);

# of outcomes when \(sum\leq{4}\) is (1,2) and (1,3), so 2.

\(P=1-\frac{2}{10}=\frac{4}{5}\).

Answer: \(\frac{4}{5}\).

P. S. In the future pleas post the answer choices too. _________________

Re: Probability clarification [#permalink]
13 Dec 2012, 00:53

sridhar wrote:

rxs005, First problem is 5C2 = 10 .. Not 20..

Point 2, you sum should be greater than 4, so these are the following possibilities you have ({1,4},{1,5},{2,3},{2,4},{2,5},{3,4},{3,5},{4,5}) which leads to 8 pairs

So the probability is 8 on 10 or 4/5..

How is the pare 1,4 GREATER than 4, I would say it is equal to four and therefore the answer should be 7/10. Any argument ?

Re: Probability clarification [#permalink]
10 Jan 2013, 20:54

arnijon90 wrote:

sridhar wrote:

rxs005, First problem is 5C2 = 10 .. Not 20..

Point 2, you sum should be greater than 4, so these are the following possibilities you have ({1,4},{1,5},{2,3},{2,4},{2,5},{3,4},{3,5},{4,5}) which leads to 8 pairs

So the probability is 8 on 10 or 4/5..

How is the pare 1,4 GREATER than 4, I would say it is equal to four and therefore the answer should be 7/10. Any argument ?

Because: 1+4 = 5 and 5>4

gmatclubot

Re: Probability clarification
[#permalink]
10 Jan 2013, 20:54

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