Let x be the total amount collected at the door. Then, we can write

where the 0.02 denotes the comission of 2%. From this equation, we can find x.
Then, by moving 3000 to the right hand side, we have

which gives

Now, by moving 0.02 to the right hand side, we get

then, x is given by

Therefore, the total amount collected at the door is $ 27 000.