What do we know?: Now we have something on spare. We need to know what we need and we don't.
We are given the diameter of the coin, the lenght of the arm (the distance from “eye” to the disk)and what we have just calculated.

What do we have to find out?: We are asked to find out the diameter of the coin.

Working out: Aren't proportional what you konw and what you want to find out?(What about cross-multiplication?)