Homework Help: Find 'k' (# of standard deviations from the mean)

1. Mar 5, 2012

rogo0034

1. The problem statement, all variables and given/known data
μ=10 σ^2=4

I seem to be getting the first two just time the same as i get every other problem, but I can't seem to get C)'s 'k'. typically i will take the absolute value of the portion that we are looking at, i.e. P(-10<X<10) would be |-10+10| = 20, divide that by two to get the center, 20/2 = 10 and then divide that by the standard deviation, which for this problem is 2. So 10/2 = 5, which i use for K. It's worked for every problem so far, but for C) they have 2.5 instead of (5/4) that i get. Any ideas?

2. Relevant equations

P(μ−kσ<X<μ+kσ)≥1− 1/k^2

3. The attempt at a solution