In still water, a boat averages 9 miles per hour. It takes the same amount of time to travel 28 miles downstream, with the current, as 8 miles upstream, against the current. What is the rate of the water’s current?

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?