Z score question

• Nov 9th 2011, 02:31 AM
realistic
Z score question
the time taken to for grade 4 students to complete a small puzzle follows a normal distribution with the standard deviation of 30 seconds. If 70% of grade 4 students complete the puzzle in 4 minutes or less, find the mean completion time for the grade.

im stuck with this one i unsure how the bell shape graph would look.
• Nov 9th 2011, 02:49 AM
realistic
Re: Z score question
• Nov 9th 2011, 02:57 AM
CaptainBlack
Re: Z score question
Quote:

Originally Posted by realistic
the time taken to for grade 4 students to complete a small puzzle follows a normal distribution with the standard deviation of 30 seconds. If 70% of grade 4 students complete the puzzle in 4 minutes or less, find the mean completion time for the grade.

im stuck with this one i unsure how the bell shape graph would look.

The z-score that corresonds to 70% is ~=0.52.

z=(4-mu)/0.5=0.52

which you may solve for mu (in minutes).

CB
• Nov 9th 2011, 03:03 AM
realistic
Re: Z score question
i have a chart the largest z-score i can get is 3.99
• Nov 9th 2011, 10:14 AM
CaptainBlack
Re: Z score question
Quote:

Originally Posted by realistic
i have a chart the largest z-score i can get is 3.99

You are doing an inverse look -up in the table, you are looking for the value of z that gives an area of 0.7, or if the table is from z=0, an area of 0.2.

CB