total ways in which u can select 1 from 1000 = 1000
total ways in which u can select 1 from 800 = 800
total ways u can select 1 junior and 1 senior = 1000 * 800
total ways u can select 1 sibiling from one class = 60
total ways u can select the same siblings pair from the other class = 1

so ans = 60*1 / 1000 * 800
= 3/40000
pls confirm

Hey u are correct? But you can you please provide more elaboration on this topic

Hi dferm,

Probability is (# of good outcomes)/(all possible outcomes). We want to find out the probability that a sibling pair is selected. Because there are 60 pairs, there are 60 possible good outcomes.

How about the total number of outcome possible? Well for each of the 1,000 juniors that can be picked, there are 800 possible ways to pick a senior. So the total number of selections possible is 1000*800

The probability we seek is 60/(1000*800) or 3/40,000. The answer is A

2 alternative ways to look at the question as well as a take-away lesson and step-by-step video solution are available at GMATprep Question 1092. To create practice drills with similar questions, set topic='combinatorics' and difficulty='600-700' in the Drill Generator

-Patrick