Some people think January 1, 2000 was the first day of the 21st century. Others think it was January 1, 2001. But everyone should agree that January 1, 2002 was the first “sum-day” of the new century- when you write out the date in numbers, 01/01/02, and 1+1=2. A sum-day is a date in which the day and month add up to the last two digits of the year. With that in mind:
A) What is the last sum-day of the 21st century?
B) How many sum-days are there in the 21st century?
Answers:
A) The last sum day is 12/31/43 or December 31, 2043
B) 365 as every day in a standard (non-leap) year is part of a sum-day for some year.
For example, November 24 is a sum-day for the year 2035, because 11+24=35. But the leap day doesn’t work because February 29, 2+29=31, but 2031 will not be a leap year.
Math Confidence by Robin the Math Lady 