Multiplying decimals by 10,100 or 1000

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?

The track team ran a mile and a quarter during their practice how many kilometers did her team runs?

Ineed to know how to write question one in mathematical terms