The rule of the simple interest is
![I=\text{PRT}](https://img.qammunity.org/2023/formulas/mathematics/college/8al5hlyqqe7wy2xbqt6s9o5c8cg4pk6wox.png)
P is the initial value
R is the annual rate in decimal
T is the time in years
Since the annual rate is 3.6%, then
![\begin{gathered} R=(3.6)/(100) \\ R=0.036 \end{gathered}](https://img.qammunity.org/2023/formulas/mathematics/high-school/b2nj3nylg1h07sg971jquei0kj4h6dgimo.png)
Since his loan is $15,600, then
![P=15600](https://img.qammunity.org/2023/formulas/mathematics/high-school/sdc7lei93wcords4528m0qpyhxt3grrb8g.png)
Since the time is 78 days, then
![T=(78)/(365)](https://img.qammunity.org/2023/formulas/mathematics/high-school/lhv0zwan7tz3m59pa9fgl0k4jrs70lcahr.png)
Substitute them in the rule above
a)
![\begin{gathered} I=15600(0.036)((78)/(365)) \\ I=120.0131507 \end{gathered}](https://img.qammunity.org/2023/formulas/mathematics/high-school/3h52zdh2nthzwgu4lefkbvyp3mq850e1o8.png)
Round it to the nearest cents (2 decimal places)
I = $120.01
The interest will be $120.01
b)
To find the amount owed after 78 days add I and P
![\begin{gathered} T=P+I \\ T=15600+120.01 \\ T=15720.01 \end{gathered}](https://img.qammunity.org/2023/formulas/mathematics/high-school/nctfaivb8ofwguj02loj1hh4qxvql398w3.png)
The amount owed after 78 days is $15720.01