Statistics Probability

So let's say you have a slot machine that has 3 reels. Each reel has 20 different positions on it. You have only 6 winnings. It must have a winner 15% of the time.
I was thinking this is how it has to be done
(6/20)^2(I did this because it is two of the same picture and any kind of picture)
The thing is that I seem to not be able to get 15%. If anyone can help that would be great.
Asked Jan 16, 2016

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