Question

The average age of all the student of a class is 18 years. the average age of boys of the class is 20 years and that of the girls is 15 years. if the number of girls in the class is 20, then find the number of boys in the class.

Answer 1

x = boys in class and y=girls in class=20.

given avg of class is ={(20x+15(20))/(20+x)}=18.

hence from this equation x will be 30 i.e.,No.of boys in class is 30

Answer 2

Answer:

The number of boys in the class is 30.

Step-by-step explanation:

Let x be the number of boys in the class,

Also, the average age of the boys = 20,  ( given )

Total sum of the ages of boys = 20x

(Since, Sum of the observation = Average × number of the observation )

Now, the number of girls = 20,

And, the average age of the girls = 15

Total sum of the ages of girls = 20 × 15 = 300,

Total students = x + 20

So, the total sum of the ages of the students ( both boys and girls ) = 20x + 300

$\text{Average age of class}=\frac{\text{Total sum of ages}}{\text{Total students}}$

$=\frac{20x+300}{x+20}$

According to the question,

$\frac{20x+300}{x+20}=18$

$20x+300=18(x+20)$

$20x+300=18x+360$

$2x=60\implies x = 30$

Hence, there are 30 boys in the class.