A very fast-moving train car passes you, moving to the right at (0.50 c). You measure its

Question:

A very fast-moving train car passes you, moving to the right at $0.50 c$. You measure its length to be $12 \mathrm{~m}$. Your friend David flies past you to the right at a speed relative to you of $0.80 c$. How long does David measure the train car to be?

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