Computations in Ms SQL

Question

Dear 

Friends

Below is a table of customers with debts in given dates. The available amount is the only amount capable to pay their debts (Food, Rent).

For example Peter has 220 000 amount to pay his debts i.e Food 40000+40000+40000+10000 and rent 30000+30000+30000 whicch is equal to 220000)

The unpaid amount then shall be food 30000 and rent 30000.

For Emma the last customer he shall be able to pay the entire debt as available amount is greater that total debt.

My question is given available data how can I compute Unpaid Food and Unpaid rent as displayed below. I am using MS SQL

Your response shall be much appreciated.

	Dates	AvailableAmount	Food	Rent	Unpaid Food	Unpaid Rent
Peter	31/01/2003	220000	40000	30000	0	0
Peter	28/02/2003	220000	40000	30000	0	0
Peter	31/03/2003	220000	40000	30000	0	0
Peter	30/04/2003	220000	40000	30000	30000	30000
Simon	31/01/2003	120000	40000	30000	0	0
Simon	28/02/2003	120000	40000	30000	0	20000
Simon	31/03/2003	120000	40000	30000	40000	30000
Simon	30/04/2003	120000	40000	30000	40000	30000
Edward	31/01/2003	80000	40000	30000	0	0
Edward	28/02/2003	80000	40000	30000	30000	30000
Edward	31/03/2003	80000	40000	30000	40000	30000
Edward	30/04/2003	80000	40000	30000	40000	30000
John	31/01/2003	85000	40000	30000	0	0
John	28/02/2003	85000	40000	30000	25000	30000
John	31/03/2003	85000	40000	30000	40000	30000
John	30/04/2003	85000	40000	30000	40000	30000
Emma	31/01/2003	280000	40000	30000	0	-
Emma	28/02/2003	280000	40000	30000	0	0
Emma	31/03/2003	280000	40000	30000	0	0
Emma	30/04/2003	280000	40000	30000	0	0