When doing ratios, it doesn't matter what the two quantities are, as long as they are of the same type. Dollars or bananas or whatever, but you can't mix dollars and bananas (unless you're buying bananas!). So it's the numbers that matter: 30:18. These numbers have a common factor which you can use to divide into each of the numbers. The common factor here is 6. 30/6=5 and 18/6=3, so the simplest form of the ratio is 5:3.