On the number line, if r < s, if p is halfway between r and s, and if t is halfway between p and r, then (s - t)/(t - r) =

A. 1/4

B. 1/3

C. 4/3

D. 3

E. 4

The book tells me that the distance from r to t is x, the distance from t to p is x and the distance from p to s is 2x.

The book concludes that

(s - t)/(t - r) = (x + 2x)/x, which of course is 3.

I do not understand the logic here.