A stress of 1000 psi is applied to a polymer that operates at a constant strain; after

Question:

A stress of 1000 psi is applied to a polymer that operates at a constant strain; after six months, the stress drops to 850 psi. For a particular application, a part made from the same polymer must maintain a stress of 900 psi after 12 months. What should be the original stress applied to the polymer for this application?