A data processing system and method are provided for allocating all expenditures, accrued interest attributable to each expenditure, and payments made by respective credit cardholders, to sub-accounts specifically designated by the cardholder. In a preferred embodiment, at the time of each expenditure, the cardholder electronically enters the sub-account to which the expenditure is to be allocated with the use of an electronic device having a keypad and display. The data processing system allocates to each designated sub-account all expenditures specified, as well as all interest charges that accrue as to each designated sub-account to which the interest charges are attributable. The data processing system further allocates payments made by the cardholder to the designated sub-account(s) according to the cardholder's instructions and calculates interest charges allocable to each designated sub-account as adjusted after each payment. The data processing system also tracks and calculates all expenditures and payments on a cumulative basis (i.e., without regard to the sub-accounts) so that monthly or other period billing statements will reflect cumulative expenditures for the billing period, and prior outstanding account balances and credits. The system also calculates and generates a breakdown of new expenditures to each sub-account for the billing period, as well as cumulative outstanding balances on each sub-account.