30÷(75%÷( 1-75%)-60%÷( 1-60%))- 10

=30÷(3- 1.5)- 10

=20- 10

= 10 (person)

Explanation: After adding 10 girls to the math interest group in a school, the number of boys is 60%←( 1-60%)= 1.5 times.

After adding 30 boys, the number of boys is 75%⊙( 1-75%)= three times that of girls.

(Adding 30 boys) corresponds to (3-10) times the number of girls after adding10 girls.

So after adding 10 girls, the number of girls is 30÷(3- 1.5)=20.

Number of female students 10 (person)

The initial number of boys is:

(10+10) x (60% ÷1-60%) = = 20x1.5 = 30 (person)