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'a' is correct.
'b' is incorrect, you've made 2 errors. First, look at what it's asking. It wants odds that all three ARE regularly engaged in exercise. Second, you've used the wrong operation. Multiplication is correct, but you are not multiplying the correct thing.
'c', well there are a lot of assumptions in that. But I'm guessing that the assumption being asked for is something along the lines of you're assuming that taking removing one person from the population is not enough to have any significant effect on the percentage of the remaining population that exercises. If, for example, the population was only 5, with the same 20% chance of a person being and exerciser, if you select one person, the odds that any one of the remaining does exercise is either 25% or 0%, depending on who you picked.
d) This is correct (though I think you may have reveresed the numbers in your head). Use this answer as a model to re-evaluate b)
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-TJ
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