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 char \u8:≤ not set up for use with LaTeX.). (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)