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. What is the number of boys in the class now?

The ratio of the number of boys to the number of girls in a class is 2 : 5.

Let's assume the number of boys to the number of girls in a class is 2y and 5y.

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.

$$\frac{2y-10}{5y+20}\ =\ \frac{4}{25}$$

$$\frac{y-5}{y+4}\ =\ \frac{2}{5}$$

5y-25 = 2y+8

5y-2y = 25+8

3y = 33

y = 11

Number of boys in the class now = $$(2y-10)$$

= $$(2\times11-10)$$

= (22-10)

= 12