One shirt costs $12.
It is marked up by 25%
Note that 25% is the same as 0.25
Step 1: multiply 12 with 0.25
12 x 0.25 = 3
Step 2: add 3 (the amount gotten by multiplying) into the original price (as it is a mark up. If it was a mark down, subtract).
12 + 3 = 15
$15 is your answer
hope this helps