#### Answer

$1.4*10^{6}$ years

#### Work Step by Step

To solve this question, you divide the number of particles by the number of particles the entire population can count in a year. To find the number of particles the entire population can count in a year, you multiply the population number by how many particles one human being can count in a year which is 2 multiplied by the number of seconds in a year (since the question states that everybody counts at a rate of 2 particles per second). The number will come out to be $6.307*10^7$, then multiply this by $6.9*10^9$ (6.9 billion). You will end up with $4.4*10^{17}$. Next divide $6.0*10^{23}$ by $4.4*10^{17}$ to get your final result. Make sure to round your answer to 2 significant digits.