5/6 is about midway through 2/3 and 1. It helps if you lay everything out in a number line like below:
0______________1/4______________ 2/4______________3/4___________1
0____________________1/3___________________2/3________________ 1
0________1/6________ 2/6________3/4_________4/6________5/6_______1
Additionally, 2/3 = .66 so the distance from 2/3 to 1 is approximately .34. If you divide that by 2 to see what the middle is you get .17, which you then add to .66.
.66 + .17 = .83
.83 = 5/6