SOLUTION: 9) The time spent by students on a particular E270 homework assignment follows a normal distribution with a mean μ = 150 minutes and standard deviation σ = 40 minutes. T
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Question 454467: 9) The time spent by students on a particular E270 homework assignment follows a normal distribution with a mean μ = 150 minutes and standard deviation σ = 40 minutes. Then 90% of student finish the assignment within ± _____ minutes from the mean time period.
a 41.6
b 51.2
c 65.6
d 78.6
10) On a different homework assignment the standard deviation of the time spent is 30 minutes. If 20% of students finish the assignment under 115 minutes, then the mean time spent on this assignment is _____ minutes.
a 157
b 153
c 146
d 140
Answer by stanbon(75887) (Show Source): You can put this solution on YOUR website!
The time spent by students on a particular E270 homework assignment follows a normal distribution with a mean μ = 150 minutes and standard deviation σ = 40 minutes. Then 90% of student finish the assignment within ± _____ minutes from the mean time period.
---
Draw the picture of a normal curve.
Center 90% around the mean.
That leaves a right and a left tail or 5%.
---
The z-value cutoffs are +/-1.645
The corresponding minute values are:
x = 1.645*40 = 65.8 minutes
------------------------------------
a 41.6
b 51.2
c 65.6
d 78.6
------------------------------------
10) On a different homework assignment the standard deviation of the time spent is 30 minutes. If 20% of students finish the assignment under 115 minutes, then the mean time spent on this assignment is _____ minutes.
---
Find the z-value with a 20% left tail.
z = -0.8416
------
Using x = zs+u
115 = -0.8416*30 + u
u = 115 + 25.25
u = 140.25 mminutes
================================
Cheers,
Stan H.
===============
a 157
b 153
c 146
d 140
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