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

$$x_n=a_1{r}^{(n-1)}$$

The first term $a_1=10mg$;

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

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

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

Therefore the 7th term is $a_7=10\times 2^{(7-1)}=640$