probability

[Var1995]

we will use baye's thm to solve this?
can someone solve it here?

[Dr. Strange]

P(X1=2|X2=1)=P(X2=1|X1=2)*P(X1=2)/P(X2=1)                ...BAYES THEOREM

NOW P(X2=1|X1=2)=0.5
P(X1=2)=0.5
P(X2=1|X1=1)=1
P(X2=1)=  P(X2=1|X1=1)*P(X1=1)  +   P(X2=1|X1=2)*P(X1=2)  =0.75

SO ANSWER IS 1/3