Sum() fucntion from .NET

[sahernandez]

I'm new one in Crystal report an I have this problem.
I try to sum a string.

15:30
01:45
05:25
24:00
----------------
46:40

How can I sum this values or where can i get some help topics of Crystal Report for .NET.
Thxs a lot
sahernandez

 

[sahernandez]

Now I know that I can use Time(field.string_time),
but how can I add this formula to my grand total summary and grand total,
Please understand I'm new Crystal Report

Cheers
sahernandez