I conclude this when I calculate the amount of money needed for a singular shirt in every "bundle" by using division.
First I calculated how much you'd have to pay for one shirt in the six-pack collection fo shirts, and I resulted in $4.25 for each shirt. The problem in number form is: 25.50÷6=4.24. By dividing the total with six you result in the singular price of the individual price of each shirt in the pack.
As for the other packs, my calculations resulted in $4.50 for the 4 shirts pack and $4.20 for the 5shirts pack.