The number line shown contains three points R, S, and T, whose coordinates have absolute values r, s, and t, respectively.  Which of the following equals the average (arithmetic mean) of the coordinates of the points R, S, and T ?

s

s+t-r

~$\frac{r-s-t}{3}$~

~$\frac{r+s+t}{3}$~

~$\frac{s+t-r}{3}$~