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?

12

Boys : Girls

  2    :   5

According to the question,

10 boys leave the class and 20 new girls join the class, then the new ratio of boys and girls in the class becomes

Boys : Girls

  4    :   25

⇒ \(\frac{2x - 10}{5x + 20}\) = \(\frac{4}{5}\)

⇒ 50x - 250 = 20x + 80

⇒ 30x = 330

⇒ x = 11,

⇒ The number of boys in the class now = 2x - 10 = 2(11) - 10 = 22 - 10 = 12.