You earn $35 for washing 7 cars. you earn $ for washing 4 cars.

There are numerous information's of immense importance already given in the question. Based on those information's the answer to the question can be easily determined.
Amount of money earned for washing 7 cars = $35
Then
Amount of money earned for washing 1 car = (35/7) dollars
                                                                       = 5 dollars
So
The amount earned for washing 4 cars = (4 * 5) dollars
                                                               = 20 dollars
So for washing 4 cars the amount earned will be $20. I hope the procedure is clear enough for you to understand.using this method you can always solve similar problems in future without requiring any outside help.

Method 1

$357 cars
$x4 cars

x=35*4/7
x=$20

method2:

we find out how much we earn for a car 35:7=$5 
for 4 cars, we will earn: $5x4=$20