A contoversial problem NO ONE in my class of 30 people got right. (Of course, that depends on how you see the problem)

A jet is flying from Honolulu to San Francisco, a distance of 2400 miles. Traveling through still air, the jet travels at 600 mph. It has a 40 mph tailwind. If the jet has an emergency, after how many hours would it be faster to go on to San Francisco than to turn back to Honolulu?

--------------My logic below-------------

When I did the problem, I assumed that the aircraft was traveling at 600 mph East, and 520 mph west. There are two ways to solve the problem, one with a distance/rate+time ratio and a guess and check way. I guessed and checked and arrived at an answer of 1.853333333... hours or 1/ 113/120 hours. Of course, that was "wrong." It seems like both the book and my "teacher" (who really is just someone who rewrites examples from the book and relies on the answer key heavily) agree that the aircraft is traveling at 640 mph. This could work, but the wording of the problem is tricky. Even more shocking, my teacher has trouble grasping the concept that a tailwind one way turns into a headwind the other way....so at first, she simply divided the distance by speed to arrive at her answer. I quickly alerted her to the wording of the question, and showed her how I arrived at the answer of 2 hours, if we do in fact use 640/560. she is now "discussing the question with other teachers." My question for HTers is....what is your understanding of the problem? Should I get credit for my "wrong" answer? Even though this problem is highly unrealistic and does not incorporate logistical factors, what should the correct answer be?

Thanks in advance for the help.