In ths case the answer is very simple.
Step 01:
Data:
p = investment = $217
r = interest rate = 6% = 0.06
I = interest = $78.12
t ( years) = Time = ?
Step 02:
Simple Interest
I = p * r * t
78.12 = 217 * 0.06 * t
The answer is:
t = 6 years
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