Excel formula help

I want to make a spreadsheet that keeps a bla as follows

Ref Amount Bal
5000
101 500 5500
102 400 5900
103 -200 5700

so far so good, but then I want to divide the balance into say 5 sections, I could easily set up 2 fields per section, one for amount, one for balance, but to keep form shorter, I want only one field for the updated balance. Iexample below

Ref Amount Bal Cat Bal A Bal B
5000 2000 3000
101 500 5500 a 2500 3000
102 400 5900 b 2500 3400
103 -200 5700 a 2300 3400

So I need to know how to make a formula that if Cat = A add balance to Bal A, If Cat = B add balance to Bal B etc.....

Thanks!!! I am very rusty on formulas!!!