Combining of two gases within an airflow

[sciencegen]

Homework Statement

not really a question, just stuck with something.
An airflow of 300 CFM has a contains 0.0387% CO2 (carbon dioxide-naturally occurring in air) flows through two ends of an rectangular open container from an exhaust fan located at the bottom.

Within the container 100% CO2 can be added through a controllable valve. This air will mix instantly with the passing .0387 300CFM airflow.

How much CFM of the 100%CO2 is required to add to the airflow to create a mixture of gases that = 0.5% CO2

Homework Equations

The Attempt at a Solution

I tried setting up a simple addition formula:

[(300CFM)/(.0387% CO2)] + [(x)/(100% CO2)] = (300+XCFM)/?

I'm not sure if this is even close to being right, even if it is...i don't remember how to find the new percentage of CO2 after adding a certain volume.

Maybe if the air was constant it would be easier for me...i just can't get my head around the whole "flowing" concept.

 

[chemisttree]

Let 'x' be the airflow of 100% CO2 added. The airflow of CO2 due to the fan is 300 CFM (ft^3/min) times 0.0387%. The total airflow after the addition of the 100% CO2 will be 300 + x. 5% of that is the concentration of CO2 after the addition. The same amount of air + 100% CO2 enters the chamber as exits. Solve for x.