I'm trying to have a percentage calculation continue down the column. I have
amount owing in A2, amount paid in B2, balance in C2, interest on that
balance in D2.  

I want to have E2 include the balance from C2 plus the interest calculated
(D2) using the interest rate in cell E1.

I can't figure out how to keep this cell (E1) constant when I copy the
formula down column E.

Thanks!