Let the number of girls = x
Then, According to the question,
number of the boys = 9 + 3x9+3x
Total students in class = 60, so
\begin{gathered}9 + 3x + x = 60 \\ = > 9 + 4x = 60 \\ = > 4x = 60 - 9 \\ = > 4x = 51 \\ = > x = 51 \div 4 \\ = > x = 12.75\end{gathered}9+3x+x=60=>9+4x=60=>4x=60−9=>4x=51=>x=51÷4=>x=12.75
On rounding off
Then, number of girls = 13