what is a independent variable of ($4b=t) if t= total cost and b= number of bottles?

Question

The students in Ms. chang's class decide to oder water bottles that advertise the walkathon. Maliik obtains two different quotes for the costs of the bottles.

Fill It Up charges $4 per bottle.

Bottles by Bob sharges $25 plus $3 per bottle.

Answer 1

Cost of N bottles from Fill It Up=$4N and from Bottles By Bob=25+3N dollars.

Comparing the two costs by subtracting: 4N-3N-25=N-25. When N=25 there's no difference. When N<25 Fill It Up is cheaper than Bottles By Bob. When N>25, Bottles By Bob is cheaper.