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Two groups of sizes 200 and 300 have means 25 and 10 and standard deviations 3 and 4 respectively. Find the variance of the combined sample of size 500.

1. 64
2. 65.2
3. 67.2
4. 64.2

Correct answer: 67.2

Solution

Combined mean is 16; substituting deviations gives a combined variance of 67.2.

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