Answer:
The possible digits are : 0, 2, 6, 4, 8
Step-by-step explanation:
Money charged by David to wash the windows of a car = $4
Let total number of cars he washed be x
Now, Total money earned by washing windows of a car = Money charged for washing all windows of one car × Total number of cars washed
⇒ Total money earned = 4 × x
So, the amount will always end in the digits which comes at the end of multiples of 4 because the amount will be always in the multiples of 4
⇒ 4 × 1 = 4 , 4 × 2 = 8 , 4 × 3 = 12 , 4 × 4 = 16 , 4 × 5 = 20 ......
So, the possible digits are : 0, 2, 6, 4, 8
using formula: 
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