IB Maths Studies SL, Simple Interest
The simple interest is given by the following formula
Where I is the amount of interest, P is the principal, r is the simple interest rate per annum and n is the time in years.
Regarding your question about the calculation of the simple interest we have that
The guidelines for doing this exercise directly to GDC Casio FX-9860G is
TVM -> F1:Simple Interest -> n=(4/3)*365=486.67, I%=5, PV=-8000 and then press SI and get your result with a positive sign which is the simple interest.
Hope these help!