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-
Correct Answer: 30
Required mean
$$\eqalign{ & = \left( {\frac{{60 \times 5 + 30 \times 10 + 20 \times 15}}{{5 + 10 + 15}}} \right) \cr & = \left( {\frac{{300 + 300 + 300}}{{30}}} \right) \cr & = \left( {\frac{{900}}{{30}}} \right) \cr & = 30 \cr} $$
$$\eqalign{ & = \left( {\frac{{60 \times 5 + 30 \times 10 + 20 \times 15}}{{5 + 10 + 15}}} \right) \cr & = \left( {\frac{{300 + 300 + 300}}{{30}}} \right) \cr & = \left( {\frac{{900}}{{30}}} \right) \cr & = 30 \cr} $$