Answer:
$11.50.
Explanation:
If there are 6 friends, and every ticket costs $27, then we need to multiply. 
6 x 27 = 162.
Then, we subtract the cost of the tickets so we can find the total cost of the souvenir t-shirts. 
231 - 162 = 69.
Since each of the 6 friends bought a souvenir t-shirt, we divide the total cost by the number of people.
 
 = 11.5.
So, each t-shirt costs $11.50.