To cross check your claim about the author, I did:
1800 miles ~ 2000 miles
5280 ft/mile ~ 5000 ft/mile
So 10M ft / 2M underpants = 5 ft/underpant. 12*5 = 60 in/underpant, which is close to 57, so I trust your statement :)
Sorry, but you Americans need to get rid of that imperial system: 1800 miles ~ 2000 km = 2,000,000 m. Each pair is around a metre. Simple, isn't it? Okay, admittedly, 1800 miles is definitely more than 2000 km...
1800 miles ~ 2000 miles 5280 ft/mile ~ 5000 ft/mile So 10M ft / 2M underpants = 5 ft/underpant. 12*5 = 60 in/underpant, which is close to 57, so I trust your statement :)
I think this sort of "back of the envelope" calculation is something that just comes by practice/habit and is not some innate ability. I love this article by Jon Bentley on the topic: http://www.csie.fju.edu.tw/~yeh/research/papers/os-reading-l...