if you leave a milk jug out of 1 day and the bacteria multiply by 2 every 20 minuets and it starts with 1 bacteria how much bacteria will you have if it started at 12 noon?
there are 3*24=72 lots of 20 minutes in a day It is one day isn't it?
The beginning and the end count to n=73
1,2,4,6,8,.....
$$\\T_n=ar^{n-1}\\\\ T_{73}=1*2^{72}\\\\$$
$${{\mathtt{2}}}^{{\mathtt{72}}} = {\mathtt{4\,722\,366\,482\,869\,645\,213\,696}}$$