Thread: 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.

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

i have a chart the largest z-score i can get is 3.99

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