Question

Suppose commute times in a large city are normally distributed and that 63.70% of commuters in this city take more than 22 minutes to commute one-way. If the standard deviation of such commutes is 6.3 minutes, what is the mean commute?

Answer 1

The mean is 24.205

Explanation:

Firstly, we need to get the z-score

From the question, the probability we have is the probability that commuters take more than 22 minutes to commute one-way

So the probability we had was;

P( x > 22)

So we need the z-score corresponding to 63.70% or simply 0.637

We can use the standard normal distribution table to get this

Mathematically, from the standard normal distribution table this is -0.35

z-score = (x- mean)/SD

-0.35 = (22-mean)/6.3

22-mean = -2.205

mean = 22 + 2.205 = 24.205