Joe painted 1/8 of the building.
Brianna painted 3/10 of the building.
Together, Joe and Brianna painted
(1/8 + (3/10) = (5/40) + (12/40) = 17/40 of the building.
Sue and Ryan painted the rest of the building.
The rest was
(1) - (17/40) = (40/40) - (17/40) = 23/40 of the building.
If Sue and Ryan split their part equally, then
each of them painted
(1/2) of (23/40) = 23/80 of the building .