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shuwa: Junior | Next Rank: 30 Posts; Posts: 12; Joined: Wed Jan 05, 2011 7:28 am

by shuwa » Wed Sep 28, 2011 10:12 am

- 33 students mean 13 years old , if 3 students leave 17, 18 , the youngest 5 years old out group. What the mean of remaining students?

- Chord and diamter of 2 cirle C, D. Will C be larger than B if chord and diameter > B?

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Source: Beat The GMAT — Quantitative Reasoning | Wed Sep 28, 2011 10:12 am

Tani: Legendary Member; Posts: 1255; Joined: Fri Nov 07, 2008 2:08 pmFollowed by: 90 members; Location: St. Louis; Thanked: 312 times

by Tani » Sat Oct 01, 2011 12:41 pm

If you have 33 students, the mean of whose ages is 13 and you remove three students, one 18, one 17 and one 5 years old, you use the average formula.

The sum of the terms divided by the number of terms = the average.
In the original case we do not know the sum, so we will use S. Our formula becomes
S/13 = 33. That tells us that S = 429.
We then subtract students age 17, 18 and 5 = total 40 years. Our new sum is 389 years, but we now have only 30 students so our new average is 389/30 = 12.967 years.

I am not sure I understand your second question.

Tani Wolff

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