দুইটি মুদ্রা একত্রে নিক্ষেপ করা হলে দুইটি টেইল পাওয়ার সম্ভাবনা কত?

If the probability of X to fail in the examination is 0.3 and that for Y is 0.2, then the probability that either X or Y fails in the examination is :

A man has $3$ coins $A, B$ & $C$. $A$ is fair coin. $B$ is biased such that the probability of occurring head on it is $2/3$. $C$ is also biased with the probability of occurring head as $1/3$. If one coin is selected and tossed three times, giving two heads and one tail, find the probability that the chosen coin was $A$