Question

Find the mean of 30 numbers if it is given that mean of ten of them is 12 and the mean of
remaining 20 is 9.​

Answer 1

$\boxed{\huge{\red{Answer}}}$

$\huge{Given:}$

$\frac{sum \: of \: 10 \: numbers}{10} = 12 \\ \longrightarrow{sum \: of \: ten \: numbers = 120}$

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$\frac{sum \: of \: 20 \: numbers}{20} = 9 \\ \longrightarrow{sum \: of \: 20 \: numbers = 180}$

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$\huge{\blue{Mean\:of\:30\:numbers}}$

$= \frac{sum \: of \: 30 \: numbers}{30} \\ = \frac{120 + 180}{30} \\ = \frac{300}{30} \\ = 10$

$\huge{Mean=10}$

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$\boxed{\huge{\red{@Devanshu1973}}}$

Answer 2

Step-by-step explanation:

mean of 10 students =12,

mean of 20 students =9,

mean of 30 students =?

$mean \: of \: 10 \: students = \frac{sum \: of \: observtions}{total \:obs. }$

$12 = \frac{sum \: ofobs.}{10}$

sum of obs.= 12×10=120

mean of 20 students,

$9 = \frac{sum \: of \: obs.}{20}$

sum of obs. = 20×9 =180

now,

mean of 30 students,

$x = \frac{120 + 180}{30}$

$x = \frac{300}{30}$

x=10

hence,

mean of 30 students =10.