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Ron and Sam take 1 hour to paint 7/12 of the living room.
Sam and Tom take 1 hour to paint 8/15 of the living room.
Ron and Tom take 1 hour to paint 9/20 of the living room.
How long will it take for Ron, Sam and Rom to paint the living room together?
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Let r be the Ron's rate of work (in parts of the living room per hour);
s be the Sam's rate of work, and
t be the Tom's rate of work.
From the condition, we have
r + s = 7/12 (1)
s + t = 8/15 (2)
r + t = 9/20 (3)
Add the equations (1), (2) and (3). You will get
2(r + s + t) = = = 94/60.
Hence,
r + s + t = 47/60.
It means that Ron, Sam and Tom will complete painting the room in hours = 1 hours. ANSWER
Solved.