In the sampling distribution of the mean 'time-to-finish' of samples of size n=25 runners in the Boston Marathon, the middle interval capturing 95%...

In the sampling distribution of the mean 'time-to-finish' of samples of size n=25 runners in the Boston Marathon, the middle interval capturing 95% of all sample means is (x1=259, x2=301). From this interval we can conclude that the standard deviation of the time-to-finish of all runners is.