This question was previously asked in

Official Soldier GD Paper: [ASC Centre No-2, South Bangalore] - 28 March 2021

- 23
- 18
- 20
- 21

Option 1 : 23

Concept used:

Average = Sum of observations / Number of observations

Calculation:

Sum of Age of 7 boys

⇒ 20 × 7 = 140

Sum of Age of six boys

⇒ 6 × 39/2 = 117

Age of 7th boy

⇒ 140 - 117 = 23 years.

∴ 7th boy is 23 years old.

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