Enigma 256: Ups and downs
6 February 2015
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From New Scientist #1403, 29th March 1984 [link]
“You’re looking fit!” I remarked to my nephew Henry.
“Thanks,” he replied. “I keep fit by running down Bill’s field then walking up.”
“Do you know your average speed?” I inquired.
“I’ll let you work it out — it’s a one digit number — while my speeds, up and down, are whole numbers. As you know, I don’t like fractions, and all these speeds are in miles per hour.”
“I don’t think you’ve given me enough information,” I complained.
“I think I will have, if I tell you the difference between my speeds, up and down the field.” Then he did so.
“Still not enough information.”
“Sorry! — I should have told you that I run down the field at less than 17 miles an hour. Now you have enough information.”
What was his average speed?