I want to display one field detailed item of sub report, horizontally

i have my customer details in main report and his pending bills in sub report. and their are three to four pending bills and i want to display all pending bills in one line.

custid custname billamt pendingbills
C0001 AAAA     1032    2020
                                  3030   
                                  5050
                                  4040

i want like following

custid custname billamt pendingbills
C0001 AAAA     1032    2020 + 3030 + 5050 + 4040

Please help me out this
                           
                     

Edited by jasvinder - 16 May 2012 at 2:48am

Thanks a lot for your kind reply.
just for 2 pending amounts after applying the formula output is like

+2020+3030+2020
+2020+3030+2020+3030+2020
i want pending amt like 2020+3030

i don't have group section in my report,
so where to display pending variables.

and which section should i suppress.

Please help me out for the same
thanks

Thanks for reply and solution.
now it is showing in one line.
but
1 it is repeating same result for the following customers also (they don't have any previous bills)
2 if the following customers have pending bills, then it adds previous customer's pending bills to it.

Please suggest me.
Thanks

Edited by jasvinder - 18 May 2012 at 6:25pm