Sorry -- but I don't know.
I am not sure with one, m or n, is an input and which one is output. But I believe the following rule can work:
To get n: 15-3, 10-2, 5-1, 0-0 . As you can see each time the number (3,2,1,0) that you have to subscribe to get n is getting smaller by one. (if m is an input, but if the input is n the same pattern holds but of subscribing you would add 3 then 2 then 1 and finally 0)
Also m is changes by 5, meaning next number is smaller by 5. And n is smaller by 4 each time. Knowing this you can predict what the number before 15 could be (15+5=20) and for n (12+4=16) - the difference between 20 and 16 (20-16=4) which follows the same pattern (its bigger by 1)
I hope it helps andI explained it clear enough
Sorry for misspells! Getting late!