Agroup of six tourists arrive at an airport gate 15 minutes before flight time, but only two seats are available. how many different groups of two can get on the plane? how many different groups of two cannot get on the plane?

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?