Ratio and proportion

Since the ratio is $3$ to $4$ the total is $3+4=7$ . So the fractional part of girls is $\dfrac{3}{7}$ . Now the number of girls is $\dfrac{3}{7}(70)=30$ .

in a math class of 70 students, the ratio of girls to boys is 3 to 4.

so, 3x+ 4x =70

or, 7x =70

or, x=10

so, 3x= (3 x 10) =30..........[the number of girls]