To find out how much Christian paid for the haircut including the tip, we first need to calculate the tip amount based on the $18 charge for the haircut.
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Calculate the tip: \[ \text{Tip} = 20% \text{ of } 18 = 0.20 \times 18 = 3.60 \]
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Add the tip to the original haircut charge: \[ \text{Total Amount Paid} = \text{Haircut charge} + \text{Tip} = 18 + 3.60 = 21.60 \]
Therefore, the total amount Christian paid for the haircut is $21.60.
The correct answer is C. $21.60.