What percent of these light bulbs will last as follows?

Question

Use the table to answer the question. Note: Round z-scores to the nearest hundredth and then find the required A values using the table.

A manufacturer of light bulbs finds that one light bulb model has a mean life span of 1020 hr with a standard deviation of 87 hr. What percent of these light bulbs will last as follows? (Round your answers to one decimal place.)

(a) at least 960 hr

 75.49%

(b) between 840 and 900 hr

? I have tried 10.56,10.06,10.1 and they have all been wrong.

Answer 1

mean = 1020

sd = 87

z= (960 -1020)/87 = -60/87 =-20/29 = about - .6897

use a z calculator or z tables to find corresponding area

between z= 0 and z=-.6897, then add that .2548 to .5 = .7548

= 75.48% chance

for 840, z = (840-1020)/87 = -180/87= -60/29= -2.06897

for 900 z = (900-1020)/87 = 120/87= -40/29= -1.37931

corresponding areas = .48073- .4161 = .0646= about 6.5%