Hey community! I was wondering if anyone could help me understand how to get to the answer here:
There are 3 possible combinations of the number of alphabets or numbers in the code:

4 A + 2N:
Total no. of possible combinations would be = 26 x 25 x 24 x 23 x 10 x 9
All of these combinations of alphabets and numbers can be arranged in multiple ways:
AAAANN, NNAAAA, NAAAAN, etc.
The total number of such arrangements would be 6!/(4! x 2!) = 15
Total unique access codes = 15 x 26 x 25 x 24 x 23 x 10 x 9 
3A + 3N:
Total no. of possible combinations would be = 26 x 26 x 24 x 10 x 9 x 8
All of these combinations of alphabets and numbers can be arranged in multiple ways:
AAANNN, NNNAAA, NNAAAN, etc.
The total number of such arrangements would be 6!/(3! x 3!) = 20
Total unique access codes = 20 x 26 x 25 x 24 x 10 x 9 x 8 
2A + 4N:
Total no. of possible combinations would be = 26 x 25 x 10 x 9 x 8 x 7
All of these combinations of alphabets and numbers can be arranged in multiple ways:
AANNNN, NNNNAA, NNAANN, etc.
The total number of such arrangements would be 6!/(2! x 4!) = 15
Total unique access codes = 15 x 26 x 25 x 10 x 9 x 8 x 7
Adding all of that up we have:
(26 x 25 x 10 x 9)(15 x 24 x 23 + 20 x 24 x 8 + 15 x 8 x 7)
(26 x 25 x 10 x 9 x 8 x 5)(3x3x23 + 4x24x1 + 3x1x7)
m(207 + 96 + 21)
324 m
This took me a good 10 minutes to solve
Can such questions come on GRE??
Nup
Thanks @vidishas99 ! so helpful .