Number Comparison Combinatorics

Mary randomly selected 3 digits from 1 to 9 and then arranged these as a 3-digit number with the bigger digit in the higher place value. Nestor also randomly selected 3 digits from 1 to 8 and followed the procedure as given above to form another 3-digit number. If the probability that the 3-digit number which Mary selected is greater than the 3-digit number of Nestor selected can be expressed in the form $\frac{A}{B}$ where $A$ and $B$ are co-prime integers, find $A+B$ .