i think we can solve it by finding permutation of each part separately.like this

ABC-123

possible number of Registration ={ permutation of ABC }* {permutation of 123)

= (26 P 3) * (10 P 3)

= 15600 * 720

= 11232000

AB-1234

possible number of registration = (26 P 2) * (10 P 4)

= 650 * 151200

= 98280000

A-12345

possible number of registration = (26 P 1) * 10 P 5

= 26 * 30240

= 786240

answer is AB-1234

is it correct to solve this problem in this way??