The mean of 5 observations is 60, the mean of 10 observations is 30 and the mean of 15 observations is 20. The mean of all the 30 observations is

(a) 20 (b) 25 (c) 30 (d) 40

The mean of 5 observations is 60, the mean of 10 observations is 30 and the mean of 15 observations is 20. The mean of all the 30 observations is

(a) 20 (b) 25 (c) 30 (d) 40

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