Question

The average of 25 numbers is 18 .if the average of first 12 numbers is 14 and that of the last 12 is 17, then the 13th numbers is:

Answer 1

ATQ, 
The average of 25 numbers is 18
We know that Average= Sum of all numbers/ No. of observations

So, 
18= Sum/25
Sum= 25* 18
Sum= 450

Sum of first 12 will be= 12* 14  (Avg* No. of observations= sum)
Sum of first 12 numbers= 168

And Sum of last 12 will be= 12*17= 204.

Let the 13th number be x

So, 168+204+x=450
372 + x = 450
x= 78

13th number is 78