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:

$t={\displaystyle \frac{1}{{\displaystyle \frac{1}{30}}+{\displaystyle \frac{1}{40}}+{\displaystyle \frac{1}{40}}}}=12\text{ h}$

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