24 hours because if she travels 25 miles in 6 hours and you multiply 25 by 4 to get 100, you need to multiply 6 by 4 in order for how long it takes her to run 100 miles. 6 X 4 = 24 hours
Or, if you're familiar with the empirical rule and the basic properties of the normal distribution, you can use the fact that approximately of the distribution lies within two standard deviations of the mean, or
which means about of the distribution lies outside this interval, or