Answer = 21 cars.
First we need to find out how many cars they have to wash in total. To do that we need to divide 300 by 5. 300÷5 = 60.
Since we know that they've collected 65% of their goal, we need to find 65% of 60. 65% of 60 = 39.
Now we know that they've already washed 39 cars, so we need to subtract 39 from their entire goal of 60 cars. 60-39 = 21 cars.
-Hope it helps!