Question

average of first three number is thrice the fourth number.if the average of four number is five.then find the fourth number

Answer 1

Let the numbers be a,b,c,d
By the problem,
(a+b+c)/3=3d
a+b+c=9d------(1)
(a+b+c+d)/4=5
a+b+c+d=20----(2)
a+b+c=20-d-----(3)
From (1)&(3),
9d=20-d
9d+d=20
10d=20
d=2.
Therefore,the fourth number is 2.