Answer: 3/4
Why:
The only way in which at least one tail will not occur is if both tosses give heads.
S=HH,HT,TH,TT
The probability of this is the probability of a single head multiplied by itself.
The probability of this is approximately 1/2, so the probability of two in a row is:
1/2 x 1/2 = 1/4
If the probability of getting no tails is 1/4, the probability of getting at least one tail is:
1 - 1/4 = 3/4
