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 $a_7=10\times2^{(7-1)}=640$