This did happen in my last flight. I consider making it a quick math quiz for any candidate I have to interview in the future (to see whether it takes him 5 minutes or 5 hours to analyze).

A few minutes after the captain announced that we were flying at forty thousand feet above the sea level, cruising at the speed of **900 km/h**, through the window we saw another airplane flying at roughly the **opposite direction**, but at a slightly lower altitude. Of course, the obvious question was asked: *How far is the other plane from us?* This is better than the typical Fermi problem. Assuming that that airplane was visible in our **90 degrees field of view** for **4 seconds** and it flew just **as fast as** our plane, I did a quick calculation and came up with an answer, the flight trajectory of that plane was **1 km apart** from ours.

The other guy was impressed, but he was not convinced. Now, since my math skills are rusty, I am sure I missed a thing or two. Now, what is your take?

(Picture rendered from Jarno’s public-domain vectorized aircraft).