Worker's performance

15 workers paint 180 m fence in 3 days. In how many days will 9 workers paint a 360 m fence? We assume that each worker have the same, constant and unchangeable performance.

Correct result:

x =  10 d

Solution:

$$\begin{gathered} r=180\mathrm{/}(3\cdot 15)=4\text{ m/d/r } \\ l=360\text{ m } \\ x=l\mathrm{/}(9\cdot r)=360\mathrm{/}(9\cdot 4)=10\text{ d} \end{gathered}$$

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