carlos paints the fence alone in 3 hours thus his rate is 1 fence in 3 hours = 1/3 fence/hour

lets say magno needs x hours to paint the rate alone or he has a rate is 1/x fence/hour

together they paint the fence is 2 hours thus their rate is 1/2 fences per hour

1/3 + 1/x = 1/2

by solving the equations we find

x = 6 hours

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Magno needs 6 hours to paint the fence by himself.