Determine the total number of quadruplets (A,B, C, D) of positive integers with A ≤ B ≤ C ≤ D ≤ 25, such that (A+B)*(C+D) is divisible by A*D – B*C, whenever A*D ≠ B*C.
Note: x refers to the
absolute value of x.
(In reply to
re: computer solution ... Do me a favor , Charlie by Ady TZIDON)
I think the formula you need is combi(25,4)+3*combi(25,3)+combi(4,2)*combi(25,2)//2+25
as there are three choices for which of 3 chosen numbers to appear twice, and there are C(4,2)/2 = 3 ways of pairing off two chosen numbers. We agree on the first term: four different numbers, and on the last: one number repeated 4 times.

Posted by Charlie
on 20110207 12:30:22 