Start by getting the fractions to have the same denominator by finding the Least Common Multiple of 5 and 8
LCM of 5 and 8 is 40
write the fractions in those terms:
3/8 = 15/40
2/5 = 16/40
add them together to get the total space painted:
15/40 + 16/40 = 31/40
The entire room space is 1, so the room still left to paint is:
1- 31/40 = 9/40
