Roll two dice. What is the probability that one die shows exactly two more than the other die? 
Two fair six-sided dice are rolled. What is the probability that one die shows exactly two more than the other die (for example, rolling a $1$ and $3$, or rolling a $6$ and a $4$)?

I know how to calculate the probabilities of each event by itself, but I do not know how to proceed with this problem. 

The probability of rolling a 1 and 3 is 1/18. Same for the probability of 2&4, 3&5, and 4&6.
So the overall probability of the dice being two apart equals 4/18 = 2/9.
