To find the cost for one shirt, we need to divide the total cost by the number of shirts.
The total cost is given as:
6 shirts for $18
5 shirts for $15
4 shirts for $12
3 shirts for $9
Dividing the total cost by the number of shirts, we can see that the cost for one shirt is:
18/6 = $3
15/5 = $3
12/4 = $3
9/3 = $3
Therefore, the cost for one shirt is $3.