Question : The average mark of 50 students in a class is 72. The average marks of boys and girls in that subject are 70 and 75, respectively. The number of boys in the class is:
Option 1: 20
Option 2: 35
Option 3: 25
Option 4: 30
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Correct Answer: 30
Solution : Given: The average mark of 50 students in the class is 72. The average mark of boys in the class is 70. The average mark of girls in the class is 75. Let the number of boys be $x$ and the number of girls be $(50-x)$. So, $(72×50) = 70x+75(50-x)$ $⇒3600 = 70x+3750-75x$ $⇒75x-70x = 3750-3600$ $⇒5x = 150$ $\therefore x = 30$ Hence, the correct answer is 30.
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