• Choice D

    We can also calculate any term in a Geometric Sequence using the Rule:

    $$x_n = a_1r^{(n-1)}$$

    The first term $a_1=10 mg$;

    The common ratio is 2: $r=2$;

    The number of terms excluding the first term is $2\ hours \times 60\ \frac{minutes}{hour} \div 20\ minutes =6$

    The total number of terms including the first term is $n=6+1=7$

    Therefore the 7th term is $a_7=10\times2^{(7-1)}=640$

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