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Answer:
12/11 = 1 1/11 hours
Explanation:
The total rate is the sum of the individual rates in rooms per hour.
Tom can paint 1/4 room per hour.
Sam and Victor can each paint 1/3 room per hour.
Their total rate is ...
1/4 +1/3 +1/3 = 11/12 . . . rooms per hour
The rate in "hours per room" is the inverse of this.
It takes them 12/11 = 1 1/11 hours to paint one room.