Remember what Chubby Checkers says: Disregarding distribution, the percentage of values laying within K standard deviations from the mean will be , for K greater than 1. Therefore, you can use Chubby's theorem to set up an equation:

And solve for K, which will give you how many standard deviations (to the left and right), a certain value is. These values will be the interval that corresponds to 75% of the data laying within K standard deviations of the mean.