I assume you have something like this:
@--------------@--------------@--------------@--------------@--------------@
7 ? ? ? ? 77
First find the distance from 7 to 77 by subtracting.
77-7 = 70
Divide that distance by 5 since there are 5 spaces, get 14.
Add 7+14, get 21. Put 21 for the first question mark:
@--------------@--------------@--------------@--------------@--------------@
7 21 ? ? ? 77
Add 21+14, get 35. Put 35 for the second question mark:
@--------------@--------------@--------------@--------------@--------------@
7 21 35 ? ? 77
Add 35+14, get 49. Put 49 for the third question mark:
@--------------@--------------@--------------@--------------@--------------@
7 21 35 49 ? 77
Add 49+14, get 63. Put 63 for the fourth question mark:
@--------------@--------------@--------------@--------------@--------------@
7 21 49 49 63 77
Add 63+14, get 77. You already have a 77 there, so you know it's correct.
Edwin