Correct Answer: 12

Solution : Given,
The ratio of the number of boys to the number of girls in a class is 2 : 5.
If 10 boys leave the class and 20 new girls join the class, then the new ratio of boys and girls in the class becomes 4 : 25
Let the number of boys in the class before be $2x$.
So, the number of girls in the class at that time is $5x$.
According to the question,
$\frac{2x−10}{5x+20}=\frac{4}{25}$
⇒ $50x - 250 = 20x + 80$
⇒ $30x = 330$
⇒ $x = 11$
Number of boys in class earlier = 2 × 11 = 22
⇒ The number of boys in the class now is 22 – 10 = 12
Hence, the correct answer is 12.