500 in 2:3 is saying that there are five different parts (2 + 3) This means that 500 needs to be split into five, which is 100. After that all you need to do is put 2 'sections' on one side and 3 on the other.
For example, 2 : 3 = 2( x 100) : 3( x 100) = 200 : 300
You add 2 and 3 together = 5 then divide 500 by 5 to get 100 you then times 100 by 2 for the first number = 200 and times 100 by 3 for the second number = 300 so the ratio = 200:300
