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

On the number line shown, is zero halfway between r and s?
<--r------s--t-->

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

i would always think about these things SPATIALLY / VISUALLY at first, and set up algebraic equations only as a "plan b". the problem with algebraic equations is that it's too easy to fall into traps.

the particular trap you've fallen into in your interpretation of (2) is that of assuming "-s" is to the LEFT of "t". there is no good reason whatsoever to make this assumption, and, what's more, at least one good reason (viz., "the gmat loves to test exactly these sorts of assumptions) not to make it.
of course, you don't need reasons to be very careful about your assumptions; that should be your default state.

if "-s" is to the right of "t", then you have
<--r-------s---t-----------(-s)-->
in which case 0 is in no-man's-land between "t" and "-s".
in this case, note that "s" is negative. also note that (-s) is positive in this case, a situation that is difficult to digest for most students.

taking statements (1) and (2) together eliminates the above possibility, leaving only the case that you have outlined.

Ans C