The odds of horse race betting

Rich Strike surprised everyone at the Kentucky Derby horse race in early May by moving from 18^th place at the half-way mark (out of 20 horses) to winning the race. Churchill Downs, the track in Kentucky where the race took place, paid out $81 for every dollar that was bet for Rich Strike to win. How do they figure that?  Is this fair?

For Adventurer members we have an editable Word docx of the activity, an Excel sheet of all of the race data, and our solutions.