I think the correct answer is 1600 because if the 10 cars weight 14000 in total, the average weight is 1400. If there is one car heavier, there is another one ligther, and the other 8 cars have the average weight of 1400. I mean:
8*1400=11200 pounds
14000-11200=2800 pounds (the total possible weight of other 2 cars, called A and B). And if the weight of one of them (i.e, A) is 1.5 the another one (B), we have the following:
A+B=2800 , if B=1,5A => A+1,5A=2800 => 2,5A=2800 => A=1120 ; B= 1120+1,5= 1680
And now: (1400*8)+1680 (Car B) +1120 (Car A)=14000
If we choose the answer 1700, we have the following: (1400*8)+1700+(1700/1.5)= 14033 pounds (and the total sum of the weight of the 10 cars is 14000). And if we choose another cuantity higher, the tota sum of the 10 cars increse more and more.
(Sorry for my english because I'm not english speaker)