Question

The average of 17 number is 51 if the average of first 9 numbers is 42 and that of the last 9 numbers is 62 find the 9th number

Answer 1

The average of 17 number is 51 

so

x/17 = 51

x = 51*17

x = 867

now

 average of first 9 numbers is 42 

so

x/9  42

x = 9*42

x = 372

and

 last 9 numbers is 62

x/9 = 62

x = 9*62

x  = 558

so the sum of first 9+9 = 18numbers is 

558 + 372

= 930

and  The sum of 17 number is 867 

so 9th number is 

930 - 867

63

:)