Step 1
Calculate the difference in Rich height and the height of the doorway
6' 67/100'' - 5 5 3/4''
= 1' 19/100''
Then add the difference to Riz height
1' 19/100'' + 6' 8 1/2''
= 7' 9/10''
The height of the doorway that would give Riz enough space is 7' 9/10''