extract from "Silver Blaze" - Memoirs of Sherlock Holmes
We are going well, said he, looking out of the window and glancing at his watch. Our rate at present is fifty-three and a half miles an hour.
I have not observed the quarter-mile posts, said I.
Nor have I. But the telegraph posts upon this line are sixty yards apart, and the calculation is a simple one.
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the question is "how many telegraph posts did holmes count?"
we know that 1 mile = 1760 yards
60 yards = 1 telegraph post
1760 yards = 1760/60 telegraph posts
= 29.33 telegraph posts
1760 yards per hour = 29.33 telegraph posts per hour, i.e.,
1 mile per hour = 29.33 telegraph posts per hour
53.5 miles per hour = 53.5 x 29.33 telegraph posts per hour
= 1569.155 telegraph posts per hour
= 1569.155/60 telegraph posts per minute
= 26.15 telegraph posts per minute (approximately)
= 27 telegraph posts per minute (approximately)
so, holmes must have counted about 27 telegraph posts.