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How many years during the twenty-first century have a square digital sum?
Details and assumptions:
The number has a square digital sum, since the sum of it's digits is ,which is a perfect square.
The answer is 16.
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1 solution
Years having a square digital sum in twenty first century are 2002,2011,2020,2007,2016,2025,2034,2043,2052,2061,2070,2059,2068,2077,2086,2095
as sum should be 4,9,16,25..... 2and 0 are common in all the years so, we need to see the sum of last two digits is equal to 2 or 7 or 14 or 23......... 23 and above numbers can not obtained by the sum of two digits so the above are the required years.