statistics
posted by Gracie .
1. The time required to complete a project is normally distributed with a mean of 80 weeks and a standard deviation of 10 weeks. The construction company must pay a penalty if the project is not finished by the dues date of the contract. If a construction company bidding on this contract wishes to be 90 percent sure of finishing by the due date, what due date (project week #) should be negotiated?

Use zscores.
Formula:
z = (x  mean)/sd
You are looking to solve for x, so fill in the rest of the formula with what you know.
Find z using ztable (problem gives you percentage). Mean = 80 and standard deviation = 10.
I hope this will help get you started. 
88.1594