Explanation: If you look carefully, you can see that the answer is the second condition as well. If one pipe did half, then the other pipe took over and that took 18 hours, that means that it equals one pipe filling the pool continuously by itself. :)
Since there are 4.5*10^2 grams of trash per day per person, we multiply that by 3.2*10^6 for everyone in the US and then multiply that by 365 to get how much in a year, which is 525600000000 or 5.256*10^11 (I think it's 11)