How can i solve this equation x-5=0.1(x+5)

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?

20 points - ! gia decides to buy square tiles to cover her kitchen floor. the tiles are 18” square tiles and cost $3.85 each. how much will it cost to tile her kitchen if the measurements are 11’ x 12’?

Me because i don't understand how to do it.: )