Probability distribution question

[Guest]

The time taken for grade 4 students to complete a small jigsaw puzzle follows a normal distribution with a 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 grade 4 students.


My working out:

s = 30
x - mean /s = z score

then were do I go from here?

 

[galactus]

Look up 0.70 in the body of the table and see what the corresponding z-score is. Set your formula equal to that and solve for mu.
Remember, consistent units. You must use s=1/2, not 30.