If you would like to know what part of the room has been painted, you can calculate this using the following steps:
3/8 of a room + 2/5 of a room = 3/8 + 2/5 = 15/40 + 16/40 = (15 + 16) / 40 = 31/40 of the room
The correct result would be: 31/40 of the room has been painted.