Assuming that 1) the APR represents the annualized interest rate without compounding and 2) the loan is amortized evenly over the 108 weeks, the payment amount should be:
PV * r 313 * .485/52
--------- = ------------------ = 4.7146
1-(1+r)^-n 1- (i+.485/52)^-52
So the total paid is 4.7146 * 104 = 490.32, 177.32 of which is interest.
If instead the rate is annualized with compounding, the weekly rate would be 1.485^(1/52) - 1
or 0.76% per week. The monthly payment would then be 4.3715, for a total paid of 454.64, 141.64 of which is interest.
However, normally the lender will give you the payment amount. Just multiply that by 104 and subtract off the principal - that will tell you the total interest paid.
country
oreurozone
tag because APR means something different in the US than what it means in many other parts of the world. Also, a 48.5% APR would be considered usurious (and likely illegal) in some parts of the world.