Fill alphabetic series with interval row

hi all..

how to solve this problem :
data :
1. a
2. b
3. c
4. d
etc..

after formula :
10. a
15. b
20. c
25. d
etc..

alphabetic is my goal .
any helps is greatly appreciated...
john m

=char(code(e11)+1)

Originally Posted by tim201110

=char(code(e11)+1)

how is work?..i have test but not working