Hello, mjoshua!

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: . .[1]

In the Psych class, 60% are girls: . .[2]

Substitute [2] into [1]: .

Therefore: .