Simply by using ?
When a question is of the form 'what is the probability that a project will take "at least" (suppose) 23 days', how do you find the probability using normal distribution tables?
Then, depending on whether the z-score is less than 0 or greater than zero, there might be some further fiddling around needed before a value can be read from the standard normal tables .....
If the Z-score is greater than zero, then you use 1 - Pr(Z < Z-score) since Pr(Z < Z-score) can be read straight from the table.
However, if the Z-score is less than 0, you actually use Pr(Z < -(Z-score)). The following example illustrates the chain of logic that's required:
Pr(Z > -1) = 1 - Pr(Z < -1) = 1 - Pr(Z > 1) = 1 - (1 - Pr(Z < 1)) = Pr(Z < 1).
All this is assuming of course that you're using a normal distribution .....
Look at this thread: http://www.mathhelpforum.com/math-he...y-answers.html
for some worked examples. I'd be astonished if your class notes or textbook did not have worked examples.