Answer:
Since this question is... well, it's not phrased as a question, and there is no context given, I can only assume it refers to the ratio between British Pound Sterlings (£) and US Dollars ($).
The ratio between them is 1 : 1.22 with £1 (British Pound) being equal to $1.22 (US Dollars).
£18 is actually $21.93. Therefore, the relationship between £18 and $20 is approximately 91% (with $20 being 91% of £18).
So the ratio of £18 to $20 would technically be 1 : 0.91, if that was your question.
I hope this helps.