Using a Texas Instruments BA II:
First, set the payments to monthly:
press [2nd] [I/Y] [1] [2] [enter] (this sets the payments to 12 per year)
press [2nd] [CPT] (to quit)
Now, compute the annual yield to maturity:
Input the facts we know:
15000 [PV]
300 [+/-] [PMT] (the payment is negative since you pay it)
0 [FV] (the loan will be paid off at the end)
60 [N] (there are 60 periods--5 years x 12 months per year)
ask the calculator to computer what we do not know:
[CPT] [I/Y]
the screen pauses for a second, then reads, I/Y = 7.420095794. So the yield to maturity is 7.42%
IMPORTANT: If you use this calculator for other courses, change the payments setting back to what you typically use. Also, press [2nd] [CE/C] to clear the memory of values for N, PV, FV, PMT, and I/Y.