Percentile Question

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theldot: Junior | Next Rank: 30 Posts; Posts: 15; Joined: Mon Nov 21, 2011 11:14 am

by theldot » Mon Jan 16, 2012 12:54 pm

Hey guys -
I don't have a specific example as I'm having trouble locating one, but there is a type of problem I need help with and could apprecaite a breakdown.

There was a question on the GMAT Prep software that was similar to something of the effect that 'Mary scored in the 70th percentile on a test in Class A and scored in the 90th percentile in Class B. What percentile was she in when considering both classes?'

It wasn't like that verbatim - but it was similar. Does anyone have a better example of what I am talking about and if so - can you show me the solution? I'm sorry I don't have anything better, I can't find it but I know I have seen this style question more than once.

Also, help with the 'Mary did better than 13 people and is in the 75 percentile of her class. How many kids did she do worse than' style would be helpful too. These are tripping me up.

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Source: Beat The GMAT — Problem Solving | Mon Jan 16, 2012 12:54 pm

GMATGuruNY: GMAT Instructor; Posts: 15539; Joined: Tue May 25, 2010 12:04 pmFollowed by: 1906 members; Location: New York, NY; Thanked: 13060 times; GMAT Score:790

by GMATGuruNY » Mon Jan 16, 2012 1:04 pm

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ronnie1985: Legendary Member; Posts: 626; Joined: Fri Dec 23, 2011 2:50 amFollowed by: 10 members; Location: Ahmedabad; Thanked: 31 times

by ronnie1985 » Wed Jan 18, 2012 12:10 am

Median is the 50th percentile and the value in the set which is in the middle of median and maximum is 75th percentile for a normally distributed set.

70th percentile means there are 70% students who got less than Mary (Normal Distribution is assumed) in Class A. 80th percentile means there are 80% students who got less than Mry in class B. If the no of students in each class is given, then percentile score can be calculated.

The question says that there were 13 people who got less than Mary in the exam and she is in the 75th percentile. If no student got same marks, then 13 = (3/4) of student population.

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