The half-life of caesium-137 is about 30 years. how long will it take a sample of caesium-137 to decay to 33% of the original amount? round your answer to the nearest year.

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?