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The average of four numbers is 15. If the average of first three numbers is thrice the fourth number, then determine the fourth number.

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The average of four numbers is 15. If the average of first three numbers is thrice the fourth number, then determine the fourth number.


1). 4
2). 5
3). 6
4). 7


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3 vote
Answered by on | Votes 3 |

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

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