The amount of sales tax on a clothing purchase is directly proportional to the purchase price of the clothing. The sales tax on a $60 sweater is $4.20. Determine the constant of variation

You are standing next to a really big circular lake. you want to measure the diameter of the lake, but you don't want to have to swim across with a measuring tape! you decide to walk around the perimeter of the lake and measure its circumference, and find that it's 400\pi\text{ m}400? m. what is the diameter dd of the lake?