Question

PLEASE HELP!!

Answer 1

Answer:

A) 99.74%

B) 633.6 hours and 986.4 hours

Step-by-step explanation:

Solution :

Given that ,

mean =  u =810

standard deviation = sigma  = 90    

P(540< x <1080 ) = P[(540-810) /90 < (x - u   ) /  sigma < (1080-810) /90 )]

= P( -3< Z < 3)

= P(Z <3 ) - P(Z < -3)

Using z table    

=0.9987-0.0013

= 0.9974

answer=99.74%

(B)

middle 95% of score is

P(-z < Z < z) = 0.95

P(Z < z) - P(Z < -z) = 0.95

2 P(Z < z) - 1 = 0.95

2 P(Z < z) = 1 + 0. 95= 1.95

P(Z < z) = 1.95/ 2 = 0.975

P(Z <1.96 ) = 0.975

z  ±1.96    

Using z-score formula  

x= z * sigma  +  u

x= -1.96* 90+810

x= 633.6

Using z-score formula  

x= z * sigma  +  u

x=1.96* 90+810

x= 986.4

633.6 hours and 986.4 hours