Question

f the life, in months, of a certain type of car battery is normally distributed with a mean of 88months and a standard deviation of 9 months, what should be the guarantee period if the companywants less than 5% of the batteries to fail while under warranty?

Answers:
A)Less than 16.13 monthsB)Less than 73.15 monthsC)Less than 64.78 monthsD)Less than 102.85 months

The Correct one is B, but I don't know how to get it. Can someone show me a step by step solution please.

Answer 1

this like your 1st question except you're going the other way with the z-score table

they give you the population percentage and want the z-score
... the table shows -1.645
... multiply by 9 to find the number of months less than 88

Answer 2

same webpage I gave you before:
https://www.jiskha.com/display.cgi?id=1524703735
click on : value from an area
enter:
area = .05
mean = 88
SD = 9
click on "below" to get 73.193

Answer 3

I meant to give you this page:
http://davidmlane.com/normal.html

Answer 4

Thank you both!