Soldier

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

Correct answer:

t =  12 h

Step-by-step explanation:

$$\begin{gathered} h=\frac{1}{30}+\frac{1}{40}+\frac{1}{40}=\frac{4}{120}+\frac{3}{120}+\frac{3}{120}=\frac{4+3+3}{120}=\frac{10}{120}=\frac{1}{12}\doteq 0.08331\text{/h} \\ t=\frac{1}{h}=\frac{1}{\frac{1}{12}}=12\text{h} \end{gathered}$$

Try another example