Basically, if you want a ratio of 3:2, add both sides together. This gives you five. If you divide 30 by five, you get 6. That means that 1/5 of 30 is 6. To get 3/5 (which is the same as saying the "three" side of a 3:2 ratio) you times 1/5 by three. This means that you have 18.
To get the "two" side of your 3:2 ration, simply times 1/5 by two. This will give you 12. Therefore, the 3:2 ratio of £30 is 18:12. You can check this by adding both sides and making sure it equals the amount you started with.
18+12=30. There you go :)