Simple interest can be calculated as

Where
A is the accrued amount (final amount)
P is the principal amount
r is the interest rate expressed as a decimal value
t is the time period
You have to calculate the interest rate of the loan Gil borrowed.
First step is to write the equation for r

For
A= $10280
P=8000
t=6

The interest rate is 0.0475, if you multiply it by 100 you get that it is 4.75%
To calculate the Interest, you have to subtract the money he borrowed (P) from the money he payd (A)

So he will pay $2280, which corresponds to an interest rate of 4.75%