Example: There is a summary for Cash Gifts and Pledge Payments for each Constituent that needs to be added together to get a Total.

Cash Gifts: Sum({@Cash}, {CnBio_SortKey})
Pledge Payments: Sum({@PledgePayments}, {CnBio_SortKey})

Total: Create a new formula: Sum({@Cash}, {CnBio_SortKey}) + Sum({@PledgePayments}, {CnBio_SortKey})