Richard P. answered 09/07/15
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The easy way to think about this problem is that, in six tosses, if only one is tails , the tails result can come up on the first toss, the second toss, the third toss, the fourth toss, the fifty toss , or the sixth toss . This is six possibilities, so the answer to the question is 6.
A more sophisticated analysis is that this problem is an example of "six choose 5" or "six choose 1"
In symbols "six choose five" is 6C5 = 6! /[5! (6-5)!] = 6
"six choose one " is 6C1 = 6!/[1! (6-1)!] = 6