If we observe that it is cloudy and raining. What is the probability that the grass is wet? The answer would be:
P(W=T|C=T,R =T) = P(W=T|R=T,S=T)*P(S=T|C=T)+P(W=T|R=T,S=F)*P(S=F|C=T)
But if we observe that the sprinkler is on and the grass is wet, then what would be the probability that it is raining? I'm not sure what would be the solution query to this problem?