A normal distribution of data has a mean of 15 and a standard deviation of 4. How many standard deviations from the mean is 25?

So the difference from the mean and the value is:

25 - 15 = 10

and now we need to know how many standard deviations of 4 are in that range, that is:

10/4 = 2.5

so there are 2.5 std deviations from the mean if the value is 25