Final answer:
If you notice that one of the $20 bills is counterfeit when a customer pays you with 3 $20 bills and 4 $1s, you should not accept the counterfeit bill.
Step-by-step explanation:
If a customer pays you with 3 $20 bills and 4 $1s and you notice that 1 of the $20 bills is counterfeit, you should not accept the counterfeit bill. Counterfeit money is illegal and accepting it can lead to legal consequences. It is important to verify the authenticity of the bills before accepting them as payment. In this case, you should return the counterfeit $20 bill to the customer and only accept the genuine bills and the $1 bills.