The midpoint between m and r would be the difference between the two divided by 2 and then added to the lower number... For example the midpoint between 10 and 30 would be the difference between them (30 - 10 = 20) and divide it by 2 (20/2 = 10) and then add it to the number you started with (10 + 10 = 20). 20 is the midpoint.
So the midpoint between m and r would be..
(r - m) /2 + m
