[求助]一道GMATPP的DS

irislong

              r         s      t

on the number line shown, is zero halfway between r and s?

1)s is to the right of zero

2)the distance between t and r is the same as the distance between t and -s

答案是B，请讲讲过程好吗？谢谢！

irislong

对不起，答案是C，不知道怎么求证的

jennyliuke

Based on item 1, S is positive, then T has to be positive too -- not sufficient

Based on 1 and 2 : r can be positive or negative but since r is on the left side of t, so t-r has to be positive

t-r = t-(-s) =t+s, then s+r=0, so 0 has to be the middle point.

irislong

谢谢你的分析！终于弄清了。