Each of A, B, C and D is a positive integer with the proviso that A ≤ B ≤ C ≤ D ≤ 20.
Determine the total number of quadruplets (A, B, C, D) such that A*B*C*D is divisible by 50.
(In reply to Solution
""In order to be divisible by 50, two of the integers must be some combination of 5,10,15, & 20. There are 6 combinations of this [4!/(2!2!)]. Once one of these pairs is picked, there are 153 combinations of the remaining integers [18!/(2!16!)]. This yields 918 total"
Where are (20,20,20,20) or (5,5,5,10) and hundreds of others you have missed???
P.S. Charlie's result is transparent and 100% correct.
Analytical verification is not so easy.