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The mean and variance of a set of 15 numbers are 12 and 14 respectively. The mean and variance of another set of 15 numbers are 14 and σ 2 respectively. If the variance of all the 30 numbers in the two sets is 13, then σ 2 is equal to-

1. 10
2. 11
3. 9
4. 12

Correct answer: 10

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