Hmm I added that because whenever I use online odds converters to find implied probability, I don't want 81.2746453% etc, I just want a simple percentage with maximum one decimal place... so it was done in consideration of the spirit in which I wanted to author this kata, rather than to add another level of difficulty.
Does the practical application not really matter much for CodeWars?

Thanks, I'll work on this. For my random tests, I will make sure any randomly generated fractions look like real betting odds you would get, so I'll set a maximum of 1000 (still probably higher than you would normally see), and make sure the other number is 9 or under, so 1/1000 or 1000/1.
Then I'll take another look at the rounding and get cracking on those tests. Thanks for all the suggestions!

Derp, sorry, should be rounded up.
What kind of tests does it need for rounding?
I'm currently creating random tests, but this is a kata in and of itself!

5 rounding down in all cases.