Supermarket cashiers

When at the supermarket are opened only 2 cash people waiting in the front approximately 12 minutes. How many will shorten the average waiting time in a front where supermarket open another three cashiers?

Correct result:

x =  7.2 min

Solution:

$$\begin{gathered} 2:(2+3)=d:12 \\ d=12\cdot {\displaystyle \frac{2}{2+3}}={\displaystyle \frac{24}{5}}=4.8\text{ min } \\ x=12-d=12-4.8={\displaystyle \frac{36}{5}}={\displaystyle \frac{36}{5}}\text{ min}=7.2\text{ min} \end{gathered}$$

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