Question

The mean weight of $150$ students in a certain class is $60$kg. The mean weight of boys in the class is $70$kg and that of girls is $55$kg, then number of boys and girls respectively are

• A. $100,50$
• B. $50,100$
• C. $75,75$
• D. $60,90$

Answer 1

The correct option is B $50,100$

Let the number of boys be denoted by $b$ and number of girls be denoted by $g.$
Therefore
$(b\times 70)+(g\times 55)=(b+g)60\Rightarrow 70b+55g=150\times 60\Rightarrow 70b+55g=9000...\left(1\right)$
Also $b+g=150...\left(2\right)$
Solving $\left(1\right)$ and $\left(2\right)$, we get
$b=50,g=100$