Given amount remaining = 164\frac{1}{64}641
= 164=1(2)6\frac{1}{64} = \frac{1}{(2)^6}641=(2)61
∴\therefore∴ = 120years6\frac{120 \text{years}}{6}6120years = 20 years
⇒\Rightarrow⇒ T12\frac{1}{2}21 = 20 years
