Question 595533
Hi, there!
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Odds differ from probability in that odds are a part-to-part comparison, and probability is part-to-whole.
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The odds against an event is the ratio of the number of ways the outcome cannot occur to the number of ways the outcome can occur. In this case, the event is "a horse winning a race.) The ratio 8:5 means that for every 8 times the horse does not win, there will be 5 times it does win.
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Probability is usually given as a fraction. It is a comparison of the number of ways the event we are interested in can occur to the total number of events. Here, we are comparing the ways the horse can win to the total number of races the horse enters. 
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The odds indicate that the horse will win 5 times for every 8 it does not win--or win 5 out of every 13 races. So the probability of the horse winning is 5/13
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Hope this helps! Feel free to email me if this explanation is confusing.  
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Ms.Figgy
math.in.the.vortex@gmail.com