Billy left home at 9 a. m. and rode his bicycle to the park at an average speed of 10 miles per hour. he arrived at the park at 9: 30 a. m. at this rate, how long would it take billy to ride 4 miles

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?