First, find the z-score:
z = (value - mean) / sdev
= (275 - 280) / 15
= - 0.33
In order to use a standard normal table, we need a positive z-score:
P(z < -0.33) = 1 - P(z < 0.33)
Looking at the table, we find P(z < 0.33) = 0.6293
Therefore:
P(z < -0.33) = 1 - 0.6293 = 0.3707
Hence, you have a probability of about 37% that a randomly selected pregnancy lasts less than 275 days.
