Find the indicated positive z-score: P(-z<Z<z)=.9 (a) .8159 (b) 1.28 (c) 2.575 (d) 1.645

Answer 1

P(-z < Z < z) = 0.9. Since it is the middle 0.9, you are looking for z-score whose positive and negative values corresponding to probabilities 0.5 and 0.95. Since normal distribution is symmetric, just need to look at z-score for probability corresponding to 0.95 which is 1.645 (d).