Simplify: 1/b(a-b) + 1/a^2-ab

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?

Will give a: 122 b: 90 c: 48 d: 180

Each unit cost 14p, how much would 942 units cost?