The results of a national survey showed that on average, adults sleep 6.9 hours per night. Suppose that the standard deviation is 1.1 hours and that the number of hours of sleep follows a bell-shaped distribution. If needed, round your answers to two decimal digits. If your answer is negative use “minus sign”.
(a) | Use the empirical rule to calculate the percentage of individuals who sleep between 4.7 and 9.1 hours per day. |
Enter your answer as a percentage. | |
% | |
(b) | What is the z-value for an adult who sleeps 8 hours per night? |
(c) | What is the z-value for an adult who sleeps 6 hours per night? |
Solution :
Given that ,
mean = =6.9
standard deviation = = 1.1
(A)P(4.7< x < 9.1) = P[(4.7 - 6.9) /1.1 < (x - ) / < (9.1- 6.9 ) /1.1 )]
= P(-2< Z < 2)
= P(Z <2)+P(Z <-2 )
using empirical rule
=95%/2 + 95%/2
=47.52%+47.52%
=95%
(B)using z - score formula
= (x - ) /
=(8 -6.9)/1.1
=1
z=1
(C)
using z - score formula
= (x - ) /
=(6 -6.9)/1.1
= - 0.82
z= - 0.82
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