I have a situation where I am creating a borrowing base for a company.  (I am
going to shorten numbers to make it easier.)  
There is a $300 loan.
Type A loan can borrow up to $300.
Type B loan can borrow up to $150.
Type C loan can borrow up to $50.

I need a formula to input in excel that will give me a borrowing base for
type B and C loans.  For example, to borrow in type B loans, I have to check
1-what's outstanding on the entire loan.
2-what's outstanding on type B loans.
3-Take those two numbers and figure out what can be advanced under the
answer to those questions. (I.E. $200 outstanding, $100 - type A, $100 type
B, how much is remaining in type B to borrow?)  I need a function that will
easily do that for me.  What function would I use? IF, OR, or something else?
I have a column with function for total loan availability, and total
borrowed under each loan type.  If someone could help me with the logistics
for this scenario, I would greatly appreciate it!!
Thanks in advance,
Dan