s1 normal distribution

If the mean is M and the sd is K then if 25 is less than M you say

z = (M - 25)/K

if 25 is greater than M you say

z = (25 - M)/K

once you have the z value look up the area associated with it...this will be a decimal between 0.5 and 1... let us say 0.6281

this is the area to the right of 25 in the first case ( where 25 is less than the mean ) and to the left of 25 in the other case...

Since you were asking for p( X <= 25) you need to subtract the result from 1 in the first case, but you already have the answer in the second case