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.