Question

The average of 25 observations is 18. The average of the first twelve observations of them is 14 and that of the last twelve is 17. What is the value of the thirteenth observation?
1. 74
2. 76
3. 78
4. 80

Answer 1

Correct Answer - Option 3 : 78

Given:

Total number of observation = 25

Average of 25 observations = 18

Average of first 12 observations = 14

Average of last 12 observations = 17

Formula used:

Sum of observation = Average × Number of observation 

Calculation:

Sum of all 25 observations = 18 × 25 = 450

Sum of first 12 observations = 14 × 12 = 168

Sum of last 12 observations = 17 × 12 = 204

Thirteenth observation = 450 - (168 + 204)

⇒ Thirteenth observation = 450 - 372 = 78

∴ The value of thirteenth observation is 78.