a.) x = # of bottles
Fill It Up, Inc.: y = 4x
Bottles by Bob, Co.: y = 3x + 25
b.) x is the independent variable, and y is the dependent variable because it depends on the value of x. See picture below for graphs.
c.) Not sure if the class size is given, but if there are less than 25 students then Fill It Up, Inc. would be cheapest. However, if there are more than 25 students then Bottles by Bob, Co. would be cheapest. This is shown in the graph. The class size influenced the decision.
d.) The intersection in the graphs at point (25, 100) shows that for 25 bottles the price will be the same ($100).