The Solution:
The number of fish sold in 2005 is 1,320,000,000
The number of fish sold in 2006 is 4,220,000,000
Adding the two numbers above together, we have
[tex]1,320,000,000+4,220,000,000=5,540,000,000[/tex]Thus, the best estimate of the number of fish in both years (2005 and 2006) is 5,540,000,000 (option C)