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If this plane is expensive at 2000 hours at $20K...but a good deal if it had 1000 hours -- then half an engine job is worth $5K??

I would think that there should be a linear relationship between hours and dollars -- given a fresh engine is $15K.

Yes, there's a linear relationship between hours and dollars. But I don't see how you deduced from Jeff's statment that half an engine job is worth $5K.

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