The average of four numbers is 15. If the average of first three numbers is thrice the fourth number, then determine the fourth number.
Let the numbers be ‘a’, ‘b’, ‘c’ and ‘d’
Given, (a + b + c + d)/4 = 15
⇒ a + b + c + d = 15 × 4 = 60 ----(1)
Now,
Average of a, b and c = 3 times d
⇒ (a + b + c)/3 = 3d
⇒ a + b + c = 9d ----(2)
Substituting equation (2) in equation (1)
⇒ 9d + d = 60
⇒ 10d = 60
⇒ d = 60/10 = 6
∴ Fourth number = d = 6
10. If a,b,c,d,e are five consecutive odd numbers,their average is