[TLynn]

I have been out of school for 25+ years and trying to understand how to solve certain equations. How would I find the answer to this?

A solvent is evaporating at the rate of 0.9 ft³ in a room having dimensions of 20x15x10. The ventilation rate in the room is 1500 ft³/min. Assuming STP conditions, what is the concentration of the solvent vapor in a room after 2 hours?

[Enthalpy]

Welcome, TLynn!

The unit of the evaporation rate doesn't fit.

[TLynn]

The correct answer is showing 600 PPM but I have no idea or don't understand how that was obtained.

[billnotgatez]

There is problems with the way the question was written maybe due to typos

Firstly -- (as suggested by @Enthalpy)

...  rate of 0.9 ft³⁺  ...

Rate
usually implies a time component
Either
you miss typed the question
the original source of the problem has a typo
or you are to presume a time span for some reason.

we would have expected
per hour or per minute

[TLynn]

I think I did forget the min wording after the 0.9ft³

A solvent is evaporating at the rate of 0.9 ft³/min in a room having dimensions of 20x15x10. The ventilation rate in the room is 1500 ft³/min. Assuming STP conditions, what is the concentration of the solvent vapor in a room after 2 hours?

[billnotgatez]

In general
What is the definition of PPM?

[Borek]

There are two processes - one increases the amount of solvent in the air, the other lowers it. Each can be described using a differential equation. Processes are independent, so you can assume final effect is a linear combination of both.