A direct conditional-probability computation

Suppose $P(x,y)$ denotes the joint probability mass function of two random variables X and Y where: $P(0,0)=0.4, P(0,1)=0.2,\ P(1,0)=0.1, P(1,1)=0.3$. What is the conditional probability of $X=0$, given that $Y=1$?