Answer: 2.9 hours approximately
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Step-by-step explanation:
Let's say the room has walls and ceiling of 350 sq ft in total area.
Sally can paint this area in 5 hours when working alone, so her unit rate is 350/5 = 70 sq ft per hour.
Steve needs 7 hours to do the same job when working alone, so his unit rate is 350/7 = 50 sq ft per hour.
Assuming the two people can work together without hindering each other (i.e. without getting in each other's way), then the two unit rates combine to 70+50 = 120 sq ft per hour.
Divide the total area over this combined unit rate
350/120 = 2.91667 approximately
This rounds to 2.9
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Alternatively, you can solve the equation
And you should get roughly x = 2.9