Scores on Professor Combs Statistics Final Exams have a long term history of being normally distributed with a mean of μ=70 and a standard deviation of σ=8
a.) Find the probability that a single student will score above a 75 on the Final exam.
b.) Find the probability that a single student will score between a 65 and 75 on the Final exam.
c.) Find the probability that an entire class of 20 students will have a class average above a 75 on the exam.
d) Find the probability that an entire class of 20 students will have a class average between 65 and 75 on the Final exam.
Question #7
The average age of a Bunker Hill CC student is 27 years old with a standard deviation of 4.26 years.
Assuming the ages of BHCC students are normally distributed:
a.) What percentage of students are at least 33 years old?
b.) How old would a student need to be to qualify as one of the oldest 1% of students on campus?
Nancy is a waitress at Seventh Heaven Hamburgers. She wants to estimate the average amount each table leaves for a tip. A random sample of 5 groups was taken and the amount they left for a tip (in dollars) is listed below:
$13.00 $8.00 $6.00 S3.00 $5.00
a) Find a 90% confidence interval for the average amount left by all groups. (*round to the nearest cent*)
b.) If the sample size were larger, with everything else rernaining the sane, would the margin of Error increase or decrease?
c.) If the Confidence level were 95% instead of 90%, would the range (size) of the Confidence Interval be larger or smaller?
a)
µ = 70
σ = 8
right tailed
P ( X ≥ 75 )
Z = (X - µ ) / σ = ( 75.00
- 70 ) / 8
= 0.625
P(X ≥ 75 ) = P(Z ≥
0.625 ) = P ( Z <
-0.625 ) =
0.2660(answer)
excel formula for probability from z score is
=NORMSDIST(Z)
b)
µ = 70
σ = 8
we need to calculate probability for ,
65 ≤ X ≤ 75
X1 = 65 , X2 =
75
Z1 = (X1 - µ ) / σ = ( 65
- 70 ) / 8
= -0.6250
Z2 = (X2 - µ ) / σ = ( 75
- 70 ) / 8
= 0.6250
P ( 65 < X <
75 ) = P ( -0.625
< Z < 0.625 )
= P ( Z < 0.625 ) - P ( Z
< -0.625 ) =
0.73401 - 0.265986 =
0.4680(answer)
excel formula for probability from z score is
=NORMSDIST(Z)
c)
µ = 70
σ = 8
n= 20
X = 75
Z = (X - µ )/(σ/√n) = ( 75
- 70 ) / ( 8 /
√ 20 = 2.80
P(X ≥ 75 ) = P(Z ≥
2.80 ) = P ( Z <
-2.795 ) =
0.0026
(answer)
excel formula for probability from z score is
=NORMSDIST(Z)
d)
µ = 70
σ = 8
n= 20
we need to calculate probability for ,
65 ≤ X ≤ 75
X1 = 65 , X2 =
75
Z1 = (X1 - µ )/(σ/√n) = ( 65
- 70 ) / ( 8 /
√ 20 ) =
-2.80
Z2 = (X2 - µ )/(σ/√n) = ( 75
- 70 ) / ( 8 /
√ 20 ) = 2.80
P ( 65 < X <
75 ) = P ( -2.80
< Z < 2.80 )
= P ( Z < 2.80 ) - P ( Z
< -2.80 ) =
0.9974 - 0.0026 =
0.9948
(answer)
excel formula for probability from z score is
=NORMSDIST(Z)
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