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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 Answer: Option C
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Solution(By Apex Team)

$\begin{array}{l}\text{Required mean}\\ =\left(\Large\frac{60\times5+30\times10+20\times15}{5+10+15}\right)\\ =\left(\Large\frac{300+300+300}{30}\right)\\ =\Large\frac{900}{30}\\ =30\end{array}$

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