Question

The average of 17 numbers is 70. If the average of the first nine numbers is 45 and the average of the last nine numbers is 90, then the 9th number is ______.
1. 100
2. 75
3. 50
4. 25

Answer 1

Correct Answer - Option 4 : 25

Given:

Average of 17 numbers = 70

The average of the first nine numbers = 45

The average of the last nine numbers = 90

Formula used:

Average = Sum of all observations/Total number of observation

Calculation:

According to the question

Sum of 17 numbers = 17 × 70 = 1190      ----(1)

Sum of first 9 numbers = 45 × 9 = 405      ----(2)

Sum of last 9 numbers = 90 × 9 = 810      ----(3)

Now, Adding equation (3) and equation (2), we get

Sum of 18 numbers = 1215      ----(4)

Required number = 1215 - 1190 

⇒ 25

∴ The 9th number is 25.