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The average of 15 numbers is 90, the average of the first 7 numbers is 80 and the average of last 7 numbers is 105. Find the 8th number?
1. 45
2. 48
3. 65
4. 55

1 Answer

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Best answer
Correct Answer - Option 4 : 55

Given:

Average of 15 numbers = 90

Average of the first 7 numbers = 80

Average of the last 7 numbers = 105

Formula used:

Sum of observations = Average × Number of observations

Calculation:

Sum of all 15 number = 15 × 90 = 1350

Sum of first 7 numbers = 7 × 80 = 560

Sum of last 7 numbers = 7 × 105 = 735

8th number = Sum of all number – (sum of first 7 numbers + sum of last 7 numbers)

⇒ 1350 – (560 + 735)

⇒ 1350 – 1295

⇒ 55

The 8th number is 55

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