题目

SAT问题：Milk costs x cents per half-gallon and y cents per gallon. If a gallon of milk costs z c
Milk costs x cents per half-gallon and y cents per gallon. If a gallon of milk costs z cents less than 2 half-gallons, which of the following equations must be true?

A. x – 2y + z = 0
B. 2x – y + z = 0
C. x – y – z = 0
D. 2x – y – z = 0
E. x + 2y – z = 0
which is right?

提问时间：2020-11-11

答案

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）