ANSWER
[tex]C\text{. }\$18[/tex]EXPLANATION
To find the price of the t-shirt after using the coupon, find 10% of the price of the shirt and subtract that from the price of the shirt.
10% of the price is:
[tex]\begin{gathered} \frac{10}{100}*20 \\ \\ \$2 \end{gathered}[/tex]Now, subtract that from the price of the shirt. That is:
[tex]\begin{gathered} \$20-\$2 \\ \\ \Rightarrow\$18 \end{gathered}[/tex]That is the price of the shirt after the coupon is used. The answer is option C.