Question

Find the mean of 30 numbers, given mean of ten of them is 12 and the mean of remaining numbers is 9.

Answer 1

Let the set of ten numbers among the 30, which have their mean as 12 be = x

Similarly, let the remaining numbers (30-10=20) among the 30, which have their mean as 9 be = y

Note that:

• Mean = Sum of observations/Total observations

x/10 = 12

x = 120 __(1)

y/20 = 9

y = 180 __(2)

Add (1) and (2), divide them by 30:

x+y/30 = (120+180)/30 = 300/30 = 10

Thus, the mean of all numbers is equal to 10.

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