Using factors and multiples to figure out days of the week | Pre-Algebra | Khan Academy
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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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Wow so useful
Thanks sal
Thank you sal
instead of using division and subtracting the quotient from the original number, just use modulus (%)
What is the significant of that?
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.
What about day 5 how do u do that
Well, you can do that manually. For ex - Lets say Day 1 is Monday then Day 2 is Tuesday Day 3 is Wednesday Day 4 is Thursday and Day 5 is Friday....so Day 5 as you wanted to know is a Friday.
Days
So modulus would give you the answer, right?
You can, but it's a lot more complicated, you have to take the number of days in each month into account.
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 :)
sunday comes before monday on the calendar.
You might wanna delete this...
Oh yeah sorry. The others are correct. I was looking for the number of days, which is already given.
What happens if you divide a number that leaves no reminder when divided by 7? Do you consider that a Sunday?
what about months with 31 and 30 days? won't that cause a "shift" in this formula if you wanted to know the day?
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.
Sal what about multiples of 7? They have remainder 0 so would they be Sundays according to this example?
but what if you had just a date? would this method work on that, too?
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. ;-)
Genius
'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.
I got very confused trying to understand that at the end
Yes
yes, I just tried it with thuresday being day one
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.
Yes. what important is the remainder. If the remainder is 3, so when you start from Sunday, the day will be Tuesday.
Day one is sunday
August 1st is a Thursday.
Yes. He was simplifying it though. I have mean you could if you wanted to you can make up forvthe shift by adding 6 days'ish if its more than 365. You can also subtract 3 for febuary and so on. It was just for simplicity though.
It will be Sunday
If there is no remainder, it means it falls on the same day as the day you started. The moment i saw this video i am all, wtf, i invented my own way of counting days when i was 7 years old.
Isn't this only true for if day 1 is Monday? I just tried this method to guess the day of August 21, 2013 (which is actually wed) but didn't get Wednesday. NOTE: The the first day of the month for August 2013 is a Tuesday (August 1, 2013). How would I have guessed Aug 21st to be a wed, based of your video?
sorry the 20th would be friday
Today its thursday and the 25th of June
The first day of the week depends on what browser you use but since I use Google it's Monday btw
You meant to say 3 rows before 25. But what if the 1 st isn't a Monday?
I think Sunday should technically be 0 not 7, because you can't a remainder of 7 when you dividing by 7..
say the 1st is a Thursday, list the days of the week starting with Thursday it will be the same.
I was under the impression that I could figure out what day my birthday would fall on this year with this math, but it doesn't work like that unless I already know how many days into the year it is. Its fun trivial math strategy, but wished It could have worked to tell me what day October 10 would fall on...guess I will have to keep a calandar close by always after all, lol
I think he sounds like woodysgamertag.
sal
khan
right
You are from bangladesh right
You are the best
sal khan
Not really, you only care about days of the weeks, not about months. Be it 31 days, be it 30 days or be it 100 days.No matter how many days a month has a week always has 7 days, which repeat themselfs in a 7 day circle. Therefore the formula is correct.
It would be Saturday
nope because he said day 300 so it doesn't matter how many months are in between just how many weeks.
Yes, because having no remainder when dividing by 7 is the same as saying you have a remainder of 7.
Its a Sunday, not a Saturday. You started off with Monday. Not sunday
Cool
Dont confuse other people. You're drunk. Go home
...better yet, see "The Order of the Days" on youtube... watch?v=R3dZBCC0fYk
You probably should have used your fingers to learn how to count like the majority, because your 'self invented method' is NOT an accurate system. [:
get a better mic