According to question ,

Required average age =		20 × 15 + 25 × 24		years
		20 + 25

Required average age =		300 + 600		years
		45

Correct Option: B

According to question ,

Required average age =		20 × 15 + 25 × 24		years
		20 + 25

Required average age =		300 + 600		years
		45

Required average age =		900		years = 20 years
		45

Given that , The average age of 40 students of a class = 18 years. On admitting 20 new students to the same class, the average age of the students of the class increased by = 6 months
Total age of 20 new students = (60 × 18.5 – 40 × 18) years
Total age of 20 new students = (1110 – 720) years = 390 years

Correct Option: B

Given that , The average age of 40 students of a class = 18 years. On admitting 20 new students to the same class, the average age of the students of the class increased by = 6 months
Total age of 20 new students = (60 × 18.5 – 40 × 18) years
Total age of 20 new students = (1110 – 720) years = 390 years
∴ Their average age = 390 ÷ 20 = 19.5 years
Hence , Their average age = 19 years 6 months

Here , The present average age of a family of four members = 36 years.
Sum of the present ages of whole family = 36 × 4 = 144 years
If the present age of the youngest member of the family = 12 years
Sum of the ages of the family at the birth of youngest member = 144 – 4 × 12 = 96 years

Correct Option: C

Here , The present average age of a family of four members = 36 years.
Sum of the present ages of whole family = 36 × 4 = 144 years
If the present age of the youngest member of the family = 12 years
Sum of the ages of the family at the birth of youngest member = 144 – 4 × 12 = 96 years

Here , Total age of 40 old students = 40 × 15 = 600 years.
Total age of 40 old and 10 new students = 50 × 15.2 = 760 years
∴ Total age of 10 new students = 760 – 600 = 160 years.

Correct Option: B

Here , Total age of 40 old students = 40 × 15 = 600 years.
Total age of 40 old and 10 new students = 50 × 15.2 = 760 years
∴ Total age of 10 new students = 760 – 600 = 160 years.
∴ Required average age = 160 ÷ 10
Required average age = 16 years.

Given , The average age of 20 boys in a class = 12 years.
The average age of admitted to class 5 new boys = 7 years.

Correct Option: D

Given , The average age of 20 boys in a class = 12 years.
The average age of admitted to class 5 new boys = 7 years.