What are two ways you can tell that two ratios form a proportion?

I was just doing this this morning. Proportions mean equal. For example, here is a word problem.
Say you read 1 book in a total of 2 days. The ratio would be 1:2 or 1/2. Then lets say you read  5 books in a total of 10 days. The ratio would be 5:10 or 5/10. 1/2 and 5/10 are both equivalent or proportional. 
Hope this helps XD