Soldier

An experienced soldier digs a good trench in 30 hours. Two soldiers dug the same trench each 40 hours. How many hours will the ditch dig all three together?

Correct result:

t =  12 h

Solution:

t=1130+140+140=12 ht=\dfrac{ 1 }{ \dfrac{ 1 }{ 30 } + \dfrac{ 1 }{ 40 } +\dfrac{ 1 }{ 40 } }=12 \ \text{h}

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