# Setting up a differential equation with interest

The following is the problem that I am working on.

A bank offers an annual interest of 5% per year with continuous interest for 20 yrs. Every year, a person withdraws 12,000 from the account. If the balance equals exactly 0 at the end of the 20th year, what is the principal of the account?

To set up the differential equation is the part that I am not confident.

Intuitively speaking, I know the continuous interest formula would be

$$A=Pe^{.05t}$$