Answer:
 4%
Step-by-step explanation:
The interest earned on the investment is the difference between the end-of-year value and the initial value:
 $83.20 -80 = $3.20 . . . . simple interest per year
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The interest rate is the ratio of this to the initial investment:
 $3.20/$80.00 = 0.04 = 4% . . . . . simple interest rate per year