Working alone, Carol can paint 1 room in 4 hours. David, working alone, can paint the same sized room in 6 hours. if they work together, how long would it take them to paint that sized room?

Working alone, Carol can paint 1 room in 4 hours. David, working alone, can paint the same sized room in 6 hours. if they work together, how long would it take them to paint that sized room?