Question

SAT问题：Milk costs x cents per half-gallon and y cents per gal

Answer 1

a gallon of milk costs y cents.

2 half gallons cost 2x cents.

a gallon costs z cents less than 2 half-gallons,therefore:

y=2x-z

移项,D正确.

注：一加仑花的钱不一定是半个加仑的两倍（这就好比我们买大包的薯片一般单位净重的单价会便宜一样）