There's 36 students graduated, since the dropout rate is 10%, then it means 36 students represents 90% of the students that were in the class. We can create a rule of 3 as shown below:
[tex]\begin{gathered} \frac{90}{100}=\frac{36}{x} \\ x=\frac{36\cdot100}{90} \\ x=40 \end{gathered}[/tex]There were 40 students on the class.
