1/2 = 0.50
(0.85 - 0.50) / 2 = 0.35/2 = 0.175
0.50 + 0.175 = 0.675 or 27/40 <==
0.85 - 0.175 = 0.675 or 27/40 <==
** first, u take the difference of the numbers and divide it by 2....then u add that to 0.50 and subtract that from 0.85 and that is ur middle number