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.
: |x| refers to the absolute value
Tot= C(25,4)+25*C(24,2)+2*C(25,2)+25=20175 WRONG!
Tot= C(25,4)+25*C(24,2)+3*C(25,2)+25=20475 OK
1ST TERM: xyzv
3rd : xxyy; xyyy; xxxy (one not taken in account)
12650+6900+900+25 = 20475 correct
THANK YOU , Charlie. After getting your result it took me 5 sec to locate the" missing 300"
look forward to a problem posted by me- based on this evaluation.