The only way to get a difference of 2 is to draw a "3" and then a "5" in either order.
The probability of selecting any tile is 1/4. The probability of selecting two particular tiles in a row is (1/4)*(1/4) = 1/16 since we replaced the first tile selected.
2*(1/16) = 1/8 represents the probability of selecting a 3 and 5 in either order.
