Correct Answer - Option 4 : 12

**Given:**

The average age of 6 brothers = 15 years

The age of youngest brother = 5 years

**Formula used:**

Average = sum of the ages/number of persons

Calculations:

Sum of the 6 persons = 6 × 15

⇒ 90

When the youngest brother born the sum of the remaining 6 brothers = 90 - 6 × 5

⇒ 90 - 30 = 60

Average of the group of brothers when the younger one born = 60/5

⇒ 12 years

**∴ The average age of 6 members when younger one born is 12 years**

At the time of birth, the age of the newly born baby is 0

So, the average age of 6 boys = 60/6 = 10 years

But here, no such option is available in the official paper so we have to consider at the time of birth of the baby number of boys = 5

And then their average age was 60/5 = 12 years.

**If both 10 & 12 are available in option, then 10 will gate more priority.**