Answer: 8.4 %
Explanation:
Since, the monthly payment formula,

Where, PV is the present value of the loan,
r is the rate per period,
n is the number of periods,
Here, PV = $ 80,000
P = $ 600
n = 12 × 30 = 360
Let r be the annual rate ,
⇒

⇒

⇒

⇒

Thus, the annual rate of interest = 0.082 = 8.2 %