A watch cost $48 more than a clock. The cost for the clock is 4/7 the cost of the watch. You'll find the total cost of the two items

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?