averages topic

Question

the average of 25 students is 18 ; that of first twelve is 14 and the last twelve is 17 thirteenth result is:

Answer 1

If we let x = sum of the 1st 12 students then  x/12=14

If we let y = sum of the last 12 students then y/12=17

Let z be the 13th student then   (x+y+z)/25 =18

 

x/12 = 14  so x= 14(12)= 168   this is the sum of the first 12 students

y/12 = 17 so  y= 17(12) = 204  this is the sum of the last 12 students

 

(x+y+z)/25 = 18

(168+204+z)/25 = 18

372 + z = 18(25)

z = 18(25)-372

z = 78

 

the 13th result is 78