The ratio of boys to girls was 10.
Boys:Girls = 10:1
If there were 30 boys then let x be the number of girls.
[tex]\frac{\text{Boys}}{\text{Girls}}=\frac{10}{1}=\frac{30}{x}[/tex]Let us solve for x.
[tex]\begin{gathered} \frac{10}{1}=\frac{30}{x} \\ 10\cdot x=30\cdot1 \\ 10x=30 \\ x=\frac{30}{10} \\ x=3 \end{gathered}[/tex]So, there were 3 girls.
