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Answer:
The number of persons using the elevator at any hour is never going to be less that 15.
Step-by-step explanation:
To solves this you have to suppose that there are at least 15 persons on the elevator, and the equation is converted into an inequation:
Now you transform the inequation back to an equation to solve it:
You need to know if there is any negative solution for the equation, to do this you can use the discriminant for a quadratic equation:
In this case, you have a=1, b=-10, c=25
Since the discriminant is 0 and a<0 the equation always is going to be positive. Therefore, the number of persons using the elevator at any hour is never going to be less than 15.