There are two post offices and probability of losing parcel by P1=1/5 and P2=1/5
It is a case of conditional probability
Case 1:parcel lost in P1(1st post office) =1/5 If parcel is lost by P1, it will not reach P2 hence no probability of P2 has to be considered
Case 2: parcel lost in P2: this ensures that parcel has not been lost at P1=(1-1/5)*1/5
(1-1/5)=4/5 probability that parcel is not lost by P1
According to the given question, probability that parcel is lost by P2=
(4/5*1/5)/((1/5)+(4/5*1/5)=4/9