Each friend gets an equal share. So out of the 24 bottles, everyone gets 6, or 25% of one case.
Each bottle costs $1.25. We're being asked to make this equation in terms of cases, though, so this price needs to be multiplied by 6, since we want to know what the individual friends will need to pay. The cost will then be $7.50.
Since this is the price they need to pay per case, we can make our equation:
f(c) = 7.50c is your answer.
