x - the number of girls
y - the number of boys
\(x:y=7:4 \\
\frac{x}{y}=\frac{7}{4} \\ 7y=4x \\ y=\frac{4}{7}x\)
The total number of students is 77.
\(x+y=77 \\
x+\frac{4}{7}x=77 \\
\frac{7}{7}x+\frac{4}{7}x=77 \\
\frac{11}{7}x=77 \\
x=77 \times \frac{7}{11} \\
x=7 \times 7 \\
x=49\)
There are 49 girls in the student council.
