Final answer:
To find the price of the t-shirt after the tax is added, multiply the original price by the tax rate and add the result to the original price. The correct answer is $13.80.
Step-by-step explanation:
To find the price of the t-shirt after the tax is added, we need to calculate 15% of $12 (the original price).
15% of $12 is calculated by multiplying 0.15 and $12 which gives $1.80.
Adding the tax of $1.80 to the original price of $12 gives us a total price of $13.80.
Therefore, the price of the t-shirt after the tax was added is $13.80.