Fields go plow two tractors with various performance. The first tractor plow whole field in 20 hours, the second tractor plow whole field 30 hours déle.

How long take plow whole field with two tractors?


t =  14.286 h


t*(1/20 + 1/50) = 1

7t = 100

t = 100/7 ≈ 14.285714

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