Q. The mean of 30 given numbers, when it is given that the mean of 10 of them is 12 and the mean of the remaining 20 is 9, is equal to
A) 11
B) 10
C) 9
D) 5
Explanation
(b)
Given that, $n_{1} = 10,\ {\overline{x}}_{1} = 12,\ n_{2} = 20,\ {\overline{x}}_{2} = 9$
$\therefore\ \ \ \overline{x} = \frac{n_{1}{\overline{x}}_{1} + n_{2}{\overline{x}}_{2}}{n_{1} + n_{2}} = \frac{10 \times 12 + 20 \times 9}{10 + 20}$
$= \frac{120 + 180}{30} = \frac{300}{30} = 10$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.