What number is halfway between 1/2 and 0.85? explain

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 2then u add that to 0.50 and subtract that from 0.85 and that is ur middle number