I grew up (that's debatable) in an age without a computer (I'm old) in my pocket or within shouting distance, so I learned ways to solve problems in my head. I still do that for things I want to calculate without a computer's aid. One common question that I have a quick in-my-head solution is what day of the week a future event will occur. For this example, I'll use the question "What day of the week is October 8 (i.e., my birthday)?"
I solved that quickly using two numbers:
- Which day of the week is the current month's 7th, 14th, 21st, 28th (which I remember)?
- The number of days in each month (which I know by heart)
- 31 for Jan, Mar, May, Jul, Aug, Oct, Dec
- 30 for Apr, Jun, Sep, Nov
- 28 or 29 for Feb
- Since May has 31 days, June's 7-14-21-28 will fall on Saturday (+3)
- Since June has 30 days, July's 7-14-21-28 will fall on Monday (+2)
- Since July has 31 days, August's 7-14-21-28 will fall on Thursday (+3)
- Since August has 31 days, September's 7-14-21-28 will fall on Sunday (+3)
- Since September has 30 days, October's 7-14-21-28 will fall on Tuesday (+2)
- Knowing October 7 is a Tuesday, my birthday will fall on a Wednesday
The reason for using 7-14-21-28 is that I know that sequence by heart, but with any other, I have to do the addition, but the solution will work for whatever sequence you want to use.
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