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 = **\(P(A/B)=\frac{P(A\,\cap\,B)}{P(B)}=\frac{1/4}{3/4}=\frac{1}{3}\)**.