If x: y =a: b, prove that a: x=(a+b): (x: y)

Fiona and her friends are playing a game by guessing where a coin will land when it is randomly dropped inside the square shown below. fiona guesses that the coin is likely to land in the blue area. which explains whether or not fiona is correct and why?

Ineed ! the person who answers and shows their work on they got the answer first will be marked the !

What is always true when two whole numbers are subtracted?