Let A : Getting two tails,
B : Getting at least one tail
Sample space S = {HH, HT, TH, TT}
⇒ A = {TT},
B = {HT, TH, TT}.
A ∩ B = {TT}
P(A) = \(\frac{1}{4}\) , P(B) =\(\frac{3}{4}\) , \((A\,\cap\,B)=\frac{1}{4}\)
∴ Required probability = \(P(A/B)=\frac{P(A\,\cap\,B)}{P(B)}=\frac{1/4}{3/4}=\frac{1}{3}\).