The average age of a family of 5 members is 25 years. If the age of the youngest member be 9 years. Just before the birth of the youngest member, the average age of the family was: (in years)

A 15

B 20

C 21

D 12

E None of these

Correct Answer: Option B

According to the given information,

We know that,

Mean = (Sum of all observations)/(Number of observations)

Average age of family members = 25

(∑F + 9)/5=25

∑F + 9 = 125

∑F = 116

Average age of 4 members 9 years ago = \(\frac{{\sum F - \left( {9 \times 4} \right)}}{4} = \frac{{116 - 36}}{4}\; = \;20\) years