We know about Chebyshev's theorem, we can find minimum percentage of noise level readings within 5 s.d. of the mean by using the following formula:-
[tex] Percentage= 1 -\frac{1}{x^{2}} [/tex]
Here the value of x=5 given in the question.
So [tex] 1-\frac{1}{5^{2}} \;=\;1-\frac{1}{25}\;=\;\frac{24}{25} \;=\;0.96 [/tex]
So final answer is 0.96 or 96%.