Maxwell borrows 1,000 to repair his car at 9% interest if he pays the loan back over the next 1.5 years how much does he pay back including interest

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?