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think about the task. It isnt 100% certain that Sprinter B wins, but if Bill bets between $15.17 and $15.99 it is 100% certain that he will have more money than Jack and therefore he will win certainly.

it's like You Throwing dice. If you throw a single dice, then it can fall six ways, each of which is equally likely if the dice is true. So the probability of getting one particular value is 1/6.