At a certain high school, the Psychology class consists of 60% of the senior class.
In the Psychology class, 40% of the students are boys, and there are 90 girls in this class.
What is the total number of students in the senior class at this high school?
Let = total number in the senior class.
60% of are in the Psych class: . .
In the Psych class, 60% are girls: . .
Substitute  into : .