The first guy covers parking lots per hour, (imagine like miles per hour)

The speed at which the other guy does a parking lot is parking lots per hour

Together they work at a speed of parking lots per hour

So it comes out to:

Let's multiply everything by 30x to get:

Subtract 3x from both sides:

Divide by two:

So the partner works at a rate of parking lots per hour, so it takes him to do the job alone.