OG 11th ed hard probability question

Expert replies

Redhorsep: Senior | Next Rank: 100 Posts; Posts: 49; Joined: Thu Jun 30, 2011 2:47 pm; Thanked: 1 times

by Redhorsep » Mon Aug 29, 2011 9:36 am

Source: Beat The GMAT — Problem Solving | Mon Aug 29, 2011 9:36 am

BenchPrepGURU: Newbie | Next Rank: 10 Posts; Posts: 5; Joined: Mon Jul 18, 2011 11:49 am; Thanked: 4 times

by BenchPrepGURU » Mon Aug 29, 2011 11:20 am

Hi,

Please help me understand why my answer isn't right and please show your reasoning for arriving at the correct answer, thanks!

****
A certain junior class has 1000 students and a certain senior class has 800 students. Among these students, there are 60 sibling pairs, each consisting of 1 junior and 1 senior. If 1 student is to be selected at random from each class, what is the probability that the 2 students selected will be a sibling pair?

I did: (60/1000) x (60/800) and got 9/2000, this is in the choices but it's the trap choice!

You've calculated the probability of picking two siblings - not a sibling pair.

The probability of picking a junior with a sibling is (60/1000).
The probability of picking that junior's sibling is (1/800).

(60/1000)*(1/800) = 60/800,000 = 3/40,000

BenchPrepGURU
benchprep.com

navami: Legendary Member; Posts: 540; Joined: Sat Dec 20, 2008 7:24 pmFollowed by: 6 members; Thanked: 37 times

by navami » Mon Aug 29, 2011 12:21 pm

you can select 1 student from the pairs of sibling from upper class in 60 ways.
Now you can select the other student of the same sibling pair from the lower class in only 1 way.

so 60 * 1

Now 60*1 / 800*1000 is the ans

This time no looking back!!!
Navami

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