Use Ordinary Interest

Short Answer

Use ordinary interest:
$$\begin{array} { | l | l | l | l | l | l | l | } \hline \text { Principal } & \text { Interest } & \text { Date } & \text { Date } & & \text { Simple } & \text { Amount } \\& \text { Rate } & \text { Borrowed } & \text { Repaid } & \text { Time } & \text { Interest } & \text { Paid Back } \\\hline \$ 12,000 & 9 \% & \text { Apr 5 } & \text { Aug } 9 & \text { A } & \text { B } & \text { C } \\\hline\end{array}$$