Answer:
The interval [2.5, 12.5]
Step-by-step explanation:
<em>Tchebysheff’s Theorem states that for any distribution of probabilities if k ≥ 1, at least </em>

of the data will lie within k standard deviations of the mean.
We need then to find a k greater or equal than 1 such that

or the equivalent equation

Finding the value of k

So, applying Tchebysheff’s Theorem, the manager can assure that the number of employees that can be absent in a given day, lies in the interval
[7.5-2*2.5, 7.5+2*2.5] = [2.5, 12.5]