Question

The mean weight of $60$ students in a class is $40$ kg. The mean weight of boys is $50$ kg while that of the girls is $30$ kg. The number of boys and girls respectively are ___________.

• A. $30,30$
• B. $35,25$
• C. $25,35$
• D. $20,40$

Answer 1

The correct option is A $30,30$

Let the number of boys is $x$

So, the number of girls will be $60-x$

The sum of the weight of boys $=50\times x$

Therefore, the sum of the weight of girls $=30(60-x)=1800-30x$.

Mean weight of the total students is $40$ kg

Thus $40=\frac{50x+1800-30x}{60}$

$\Rightarrow 1800+20x=2400$

$\Rightarrow 20x=600$

$\Rightarrow x=30$

Number of boys $=x=30$

and number of girls $=60-x=30$