If there were 5 yes votes for every 4 no votes, there were 5 yes votes for every 9 total votes, so 5/9 of the 7911 votes were yes. 5/9*7911=4395.
the answer is 10:20.
Hint for the future: be sure to write in the full problem. I only knew the answer because I had to do it myself, but people need more context if they are going to help you.
