Question

Use the z-score 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 1028 h with a standard deviation of 85 h. What percent of these light bulbs will last as follows? (Round your answers to one decimal place.)

(a) at least 930 h %

(b) between 800 and 870 h %

Answer 1

solution

given that

P(x >930 ) = 1 - P(x<930 )

= 1 - P[(x -) / < (930-1028) / 85]

= 1 - P(z <-1.15 )

Using z table

= 1 -  0.1251

=0.8749

answer=87.5%

(B)

P(800< x <870 ) = P[(800-1028) /85 < (x - ) / < (700-1028) /85 )]

= P( -2.68< Z < -1.86)

= P(Z <-1.86 ) - P(Z <-2.68 )

Using z table   

=  0.0314-0.0037

=0.02777

answer=2.8%