Answer
$11.6$ years
$9.9$ years
$8.7$ years
Work Step by Step
For $r=0.06$, $t=\frac{ln2}{0.06}=11.6$ years
For $r=0.07$, $t=\frac{ln2}{0.07}=9.9$ years
For $r=0.08$, $t=\frac{ln2}{0.08}=8.7$ years
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