The volume of an Olympic size swimming pool is 660 000 gallons. 1 gallon is approximately 3790 cm^3.

1. Convert gallons to cubic centimetres and express in standard form

$660\,000\times3790=2\,501\,400\,000$
$2\,501\,400\,000=2.5014\times10^{9}$

1. Convert the pool volume from cubic centimetres to litres

From part (a), the volume of the pool is
$2.5014\times10^{9}\text{ cm}^3.$
Since $1000\text{ cm}^3=1$ litre,
$2.5014\times10^{9}\div1000=2.5014\times10^{6}\text{ litres}.$

2. Find the combined flow rate and calculate the time taken

Combined flow rate
$=8\times10^{2}+1.2\times10^{3}$
$=800+1200=2000\text{ litres per minute}.$
Time needed
$=\frac{2.5014\times10^{6}}{2000}=1250.7\text{ minutes}.$