Out of five numbers the average of first four numbers is 15, and the average of last 4 numbers is 25. If the the first number is 30, then what is the value of last number?  

Out of five numbers the average of first four numbers is 15, and the average of last 4 numbers is 25. If the the first number is 30, then what is the value of last number?   Correct Answer 70

Given:

Average of first four numbers = 15

Average of last 4 numbers = 100

First number = 30

Calculation:

Let, the five numbers are v,w, x, y, z

From above question,

Sum of first 4 numbers = (v + w + x + y) = (15 × 4) = 60        ----(1)

Sum of Last 4 numbers = (w + x + y + z) =(25 × 4) = 100       ----(2)

According to the question,

⇒ v + w + x + y = 60

⇒ 30 + w + x + y = 60

⇒ w + x + y = 30

From (2),

⇒ w + x + y + z = 100

⇒ 30 + z = 100

⇒ z = 70

∴ Value of last number is 70