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Albert Chan · (This post was last modified: 08-09-2024 11:02 PM by Albert Chan.)

(08-09-2024 02:01 PM)Thomas Klemm Wrote: Here's another example: Today is 2024-08-09 which is a Friday.
In which years of this century was September 9th also a Friday?

365 MOD 7 ≡ 1. We just need to account for leap year.

Code:

Weekday  August 9, 20??

5        24  19  13      02

4            18  12  07  01

3        23  17      06  00

2        22  16  11  05   

1        21      10  04  

0        20  15  09      

6            14  08  03

Brute force is not that hard. (blanks are due to leap year)
From first row, we have Years with August 9 = Friday