Using factors and multiples to figure out days of the week | Pre-Algebra | Khan Academy

Can you help us think of a mathematical way to determine what day of the week a certain number day falls on? You may say no....but wait! Watch this.

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[radguitar]

At 4:05, Sal said, "...you can get seven rows in it before the row that gets to twenty-five..."
I know he didn't mean it but I had to point it out.
As for days of the week, they are totally arbitrary; merely systematic in our day-to-day organization.

[OvAppolyon]

You can, but it's a lot more complicated, you have to take the number of days in each month into account.

[saadmahmood]

if you were trying to find out what the 20th day would be and assuming that sunday is day one then the 20th day would be 20 divided by 7 = 2 r 6, then the sixth day including sunday as the first day would be the 20th with 2 weeks before it so the 20th would be a saturday. I hope this helps :)

[pauluk]

'300' is plus 299 days. That's the key: recognise the problem as a subtraction and remember that you are already at '1' to start with.

People just have to memorize useful facts in a personal way sometimes, such as there are 30 days in certains months. Which months well '4-6-9-11' obviously.

Exampe: we are in August and it's Saturday 17th what day of the week is December 17th (same number notice)?

Ans: go 31+30[9]+31+30[9] so think: +3 +2 +3 +2 or 10 = 3 mod 7 Answer: Tuesday.

[ArunNarasimhan_arun]

What happens if you divide a number that leaves no reminder when divided by 7? Do you consider that a Sunday?

[leexiong]

Oh yeah sorry. The others are correct. I was looking for the number of days, which is already given.

[msclrhd]

Yes. As multiples of 7 fully divide into 7, the last row is full. This means that the last day to be written out is at the right-most entry in the table, which is a Sunday.

[MARKB]

sunday comes before monday on the calendar.

[joehhhL]

instead of using division and subtracting the quotient from the original number, just use modulus (%)

[GermanSnipe]

Sal what about multiples of 7? They have remainder 0 so would they be Sundays according to this example?

[saadmahmood]

yes, I just tried it with thuresday being day one

[tigerthebest]

Today its thursday and the 25th of June

[CarlosFaria]

It's okay to be proud of your accomplishments, love and respect ourselves first is the seed that will give birth to love and respect others. ;-)

[Tammyaway]

that is true if you set your original day as 0, but in the video he set is as a 1. Therefore a remainder of 1 would fall on the same original day. He was right when he said it's a Saturday. It's not a Sunday, I'm afraid you're mistaken.

[ThomasGiles]

So modulus would give you the answer, right?

[docazsx]

I think Sunday should technically be 0 not 7, because you can't a remainder of 7 when you dividing by 7..

[yatharthsahu]

sal
khan
right
You are from bangladesh right

[expertgamer]

The first day of the week depends on what browser you use but since I use Google it's Monday btw

[crossfireyouthsquad]

say the 1st is a Thursday, list the days of the week starting with Thursday it will be the same.

[Lukibsubekti]

Yes. what important is the remainder. If the remainder is 3, so when you start from Sunday, the day will be Tuesday.