The time required to complete a project is normally distributed with a mean of 80 weeks and a standard deviation of 10 weeks.

The time required to complete a project is normally distributed with a mean of 80 weeks and a standard deviation of 10 weeks. The contruction company must pay a pentalty if the project is not finished by the due date in the contract. If a construction company bidding on this contract wishes to be 90% sure of the finishing by the due date, what due date should be negotiated?