Got my AS level stats test soon and do not understand one of the past paper questions.

The Distances travelled to work, Dkm, by the employeed at a large company are normally distributed with D-N(30,82) ..... so mean of 30 and variance of 82?

Find the probability that a randomly selected employee has a journey to work of more than 20km.

The mark scheme says

P(D > 20) = P(Z > 20-30/8)

Where did they get the divide by 8 from? I thought it was sqrt(82)

Any help would be great thanks.