i hope it is quite clear that I and II alone are not sufficeint.
Including both I and II:
stmt II says that distance b/w t & r is same as distance b/w t and -s. now, -s can not be to the right of t because s is positive and -s would be negative. So, -s is to the left of t. So where should i put -s to the left of t, so that stmt II is in agreement. obviously, -s sud fall on r. That means point r and -s are actually one-and-same.
so, the number line is something like this:
----(-s)----------0-------------(s)------t
And yes, 0 is halfway b/w s and -s..i..e. half way b/w s and r. (remember r and -s are one-and-same)