Help with primitive calculation

[Locomotive]

ex. I have a database of customers, and the dates by which they should have paid their debts to me. I need to charge interest from those are late.

It must be based on a primitive system of DOUBLING the sum each month not paid.

1.1 - 250
1.2 - 500
1.3 - 1000....

Can someone help me do it in the expression builder?

 

[neileg]

The arithmetic is
(Principal sum) *2^(number of months)

^ means raise to the power of. This operator is one of the standard operators in Access.