The length of human pregnancies is approximately normally distributed with mean = 266 days?

and standard deviation = 16 days.

b)What is the probability that a random sample of 20 pregnancies has a mean gestation period of 260 days or less?

The answer for this is : b) Standard deviation of sample mean = / n = 16 / 20 = 3.58

z = (260 - 266) / 3.58 = -6 / 3.58 = -1.68

p(z < -1.68) = 0.0465

My question is how did they arrive at 0.465 from p(z < -1.68)