I believe that you have not put enough information in the question like what is the mean all you told us was that its with in 2.5 of the mean.
P(M-OK< X < M+Ok)>= 1- 1/k^2
P(|x-mu|>= k sigma) <= 1/k^2
so P(|x-mu|) >= 2.5 sigma) <= 1/2.5^2 = 0.16
So P(|x-mu| < 2.5 sigma) = 1-0.16 = 0.84.
b) here I presume that we are to assume normality, so we look 2.5 up
in a table of the standard normal distribution to get:
So P(|x-mu| < 2.5 sigma) = 0.9876.