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:

$10:(10+n)=(15-4) : 15 \ \\ \ \\ \ \\ 15 \cdot \ 10=(15-4) \cdot \ (10+n) \ \\ \ \\ 11n=40 \ \\ \ \\ n=\dfrac{ 40 }{ 11 } \doteq 3.636364=4$

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