grouping and subgrouping

Hi there

i really need help with my report:

in my report i have 32 work types and i have to group them into 16 work groups where 13 groups are made of single work type and 3 groups made of 3, 6 and 10 work types, so i can have SUM of working hours for each work group for each worker. Say:

ADMIN = SUM HRS (single work type) = no problems

VAC = SUM HRS (singel work type) = no problems

INTDBL and INTOT and INTREG = SUM HRS (group work) = problem!!!

i have no problem with the single work groups, but i can't figure out how to SUM hours for each work group with more then one work type. the other thing is: not each worker has hrs for each work type, some work types have hrs = NULL.

please help me!!!

thank you very much in advance

Is there another field for which you can refer to?Like a workgroup field?