For question no. 648, the vets refused the offer of the said manufacturer because if they promoted that material in their mails, the customers would be interested in buying that product but they would buy part of it from pet stores or the supermarket and not from the vets thereby reducing their net income.
For 647,
Suppose the number of truckloads of refuse that was meant to be incinerated was x last year. This year, according to the premise, the number of truckloads of refuse to be incinerated will be half of last year x/2.
Now just because the number is half doesn’t mean that the residual ash will also be half because all refuse isn’t the same. So the author is assuming that the refuse generated this year will not generate more residual ash per truckload than last year.