Let's say the cost is represented as "C" and the number of pencils is represented as "P", the constant is a given "k". Since the cost, C, varies directly with the number of pencils, P, it means that when the cost increases, the amount of pencil you can buy also increases given a particular constant. In formula form, this is C = kP. Say you plan to buy 5 pencils that cost 7 each. You'll be paying 35 for the 7 pencils. If you buy 3, you'll pay 15. There is a direct relationship. As you increase one value, the other also increases.