Question

the life expectancy of a typical light bulb is normally distributed with a mean of 2000 hours and a standard deviaton of 27 hours. what is the probability that a lightbulb will last between 1975 and 2050 hours?

a.)0.17619
b.)0.32381
c.)0.79165
d.)0.96784

Answer 1

convert 1975 and 2050 to z-scores using the formula z-score = (given number - mean)/standard deviation

then use a z-table or a calculator (normalcdf(z-score1,z-score2) to find the probability

Answer 2

Idk what that is

Answer 3

A z-table I mean

Answer 4

http://4.bp.blogspot.com/_5u1UHojRiJk/TEh9BHxxPUI/AAAAAAAAAIQ/DafeQNMYFoE/s1600/ztable.gif

Answer 5

Okay.. how would I find the z-scores?

Answer 6

use the formula I just gave you

Answer 7

Would the answer be 0.96784

Answer 8

hm, not quite, can you tell me where you got your answer?

Answer 9

I'm a little confused on how to do this all. The two answers I got were -.925925926 and 1.851851852

Answer 10

your numbers are right, you're just not done yet

1. find the probability that corresponds to -.925925926 using the z-table
2. find the probability that corresponds to 1.851851852 using the z-table

take the result from part 2 and subtract the result from part 1

Answer 11

I got .925925926

Answer 12

uh, not quite

use the z-table to find the probability that corresponds to -.925925926

Answer 13

actually, do you have a graphing calculator with you?

Answer 14

No I don't

Answer 15

alright, so for the z-score -.925925926 we're going to round to -0.93, and use this z-table to find the probability

https://www.bing.com/images/search?q=negative+z+table&view=detailv2&&&id=0EE0897D340872D7DD333EA703C06E1FAFE3D682&selectedIndex=0&ccid=UAu%2f5RDN&simid=608005827397226440&thid=JN.QiHVxQ5sExJprxixifrppg&ajaxhist=0

look at the left column and find the row -0.9, then go across to the column 0.03 and tell me what number is in the box