Talk:2013 AMC 12A Problems/Problem 20

Disucssion:

COuld someone please explain how you got the first set of conditions is $45*10-\sum_{k=1}^{9}\dbinom{k+1}{2}$? Thank you!

Retrieved from "https://artofproblemsolving.com/wiki/index.php?title=Talk:2013_AMC_12A_Problems/Problem_20&oldid=112184"
Invalid username
Login to AoPS