Chad can paint a room in 5 h. cassie can paint the room in 3 h. how long would it take them to paint the room working together?

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.

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