Probability Question

[KareemE937]

Copyright@ Jamboree Education Pvt.Ltd.2012:Question Number 196:
The probability is 1/2 that a certain coin will turn up heads on any given toss. if the coin is to be tossed three times, what is the probability that ob at least one of the tosses the coin will turn up tails?
I tried to answer the question using that it mentioned in the question at least one of the tosses the coin will turn up tails, so it might be one tail or 2 tails or 3 tails so the answer should be= 1/2+1/2+1/2= 3/2 but I found that the correct answer is 7/8, so how the right answer is 7/8?

[RonPurewal]

"copyright jamboree education", hmm?
i wonder how GMAC will feel about that. because, you know, this problem is exactly the same as OG PS #193. they didn't change even a single word.

[RonPurewal]

in any case, since this is an OG problem (and is thus not allowed here), this thread is locked.