There are 18 nickles so that number is 18. When added together, there are 36 total coins. Since it states nickle to all coins, we replace <em>to</em> with a colon and replace the words with the related numbers.
18:36
like a fraction, ratios can be simplified. 18 is half of 36 so the ratio would go to 1:2. This means that 1 in every 2 coins is a nickle.