6 litres per minute 6\times60 per hour 6\times60\times24 per day 6\times60\times24\times365.25 per year 6\times60\times24\times365.25\times20,000,000 total
This can be approximated by: \begin{align}6 \times 60 \times 20 \times 400 \times 20,000,000 =& 2,880,000\times 20,000,000\\
\approx& 3,000,000 \times 20,000,000\\
&=60,000,000,000,000\end{align}