Explanation:

Number of students in the class = 50

Ratio of Boys : Girls = 4 : 1

Let x be the constant ratio

Ratio of Boys : Girls = 4x : 1x

Sum of their age is 50

4x + 1x = 50

5x = 50 = x = 10
Find the number of boys and girls:

girls = x = 10

boys = 4x = 4(10) = 40
Find the average of the boys' age:

average = 2 x number of girls = 2(10) = 20
Find the sum of the boy's age:

Average of the age of the 40 boys = 20

sum of the age of the 40 boys = 20 x 40 = 800 years

Hence, option 3 is the correct answer.