Mathematics, applications of mathematics to life in general, and my life as a mathematician.
May 5 of 1955 can be written as 5/5/55. How many times during the 20th century the date in the format month day and the last two digits of the year can be written with the same digit?
N.n.nn -9 dates, 11.1.11, 1.11.11, 11.11.11, 2.22.22.
Thirteen times. Four with only the digit 1, two with only the digit 2, and one each for the digits 3 through 9.
I think there’s more – something like 6/9/96 is all written with the same digit (although I’ve had to turn the paper upside down occasionally).
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