The depth of the snow in my yard is normally distributed, with a mean of 2.5 inches and a standard deviation of 0.25 inches. what value is one standard deviation below the mean?

If you need to go one standard deviation below 2.5, all you have to do is subtract .25 from 2.5

   2.50
 - 0.25

   2.25

It helps to add zeros if the different decimal points kind of confuse you