Ten cashiers

Ten cashiers are open at Tesco. Customers wait an average of 15 minutes. How many other cashiers have to open to reduce the waiting time by 4 minutes?

Correct result:

n =  4

Solution:

$$\begin{gathered} 10:(10+n)=(15-4):15 \\ 15\cdot 10=(15-4)\cdot (10+n) \\ 11n=40 \\ n={\displaystyle \frac{40}{11}}\doteq 3.636364=4 \end{gathered}$$

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