Answer: f(x) = 1.19x, where f(x) is the total cost, and x is the cost per bottle
Cost of 15 bottles: $17.85
Explanation:
This question describes a straightforward relation between cost per bottle, and cost for all the bottles. As such, simply equate one variable to the value of one bottle, and f(x) as the value of every bottle.
Hope it helps :) and let me know if you want me to elaborate.
P.S. f(x) can simply be substituted for "y"