2011 AIME II Problems/Problem 6

Define an ordered quadruple (a, b, c, d) as interesting if $1≤a<b<c<d≤10$ (Error compiling LaTeX. ! Package inputenc Error: Unicode character ≤ (U+2264)). (Okay, if you go to edit page you can see that those wierd a's are supposed to be "less than or equal to" signs)