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SD of two groups

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by maihuna » Wed Jan 28, 2009 9:28 am
A group of 20 Bellerbys students has an average (arithmetic mean) IQ of 145 with a standard deviation of 13. A group of 15 students from a rival college (boo!) has a mean IQ of 120 with a standard deviation of 20. The students are merged into one group. What is the standard deviation of the entire group.

A 19.75
B 20.12
C 20.35
D 20.43
E 21.12

not an std question
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by Brent@GMATPrepNow » Wed Jan 28, 2009 10:21 am
This question is way out of scope for the GMAT.
To begin, it requires a very intimate knowledge of the formula for calculating SD and, more importantly, it requires a calculator.

For the solution (and to see where the question originated) see: https://richardbowles.tripod.com/maths/n ... /norm2.htm
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