If Chad can paint the whole room in 5 hours, in 1 hour he gets 1/5 of it painted. If Cassie can paint the whole room in 3 hours, in 1 hour she gets 1/3 of it painted. If we add those together we can find out how much of the room they can paint together in one hour. 1/5 + 1/3 = 1/x, with x being the number of hours it will take them to paint the room together. Common denominator is 15, therefore, 3/15 + 5/15 = 1/x and 8/15 = 1/x. Cross-multiply to get 8x = 15, so it will take them 15/8 hours, or 1.9 hours to paint the whole room together.
