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 Posted: Tue Jan 17, 2012 6:41 pm

Joined: Thu May 26, 2011 8:37 pm
Posts: 577
Hi

This week's maths homework seems complicated - all about working out which day of the week a 21st birthday will fall on if a boy was born on 13th July 2008.

I know we can look it up on i-cal (and did!) but I'm sure someone on here gave a stunningly simple piece of calculus for this at some point. I googled but only found very complex calculus so wondered if I'd misremembered.

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 Posted: Tue Jan 17, 2012 8:35 pm

Joined: Sat Feb 13, 2010 11:02 pm
Posts: 420
Was the day of the week that he was born on given?

If so,
1. then the number of days can be calculated by 21*365+5.
2. Divide the total by seven,
3. What ever the remainder is count on that many days of the week.

I think this would work, but I have not worked it out or checked it. (Sorry)

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 Posted: Wed Jan 18, 2012 12:50 am

Joined: Wed Apr 13, 2011 10:25 pm
Posts: 463
999 mum - Could you please explain why an additional 5 is added to number 1.

(Can you tell maths isn't my strongest point)

_________________
;D

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 Posted: Wed Jan 18, 2012 2:14 pm

Joined: Sat Feb 13, 2010 11:02 pm
Posts: 420
Thingsbehindthesun wrote:
could you please explain why an additional 5 is added to number 1.

the extra day in leap years.

Last edited by 999 mum on Wed Jan 18, 2012 3:25 pm, edited 1 time in total.

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 Posted: Wed Jan 18, 2012 3:24 pm

Joined: Sat Feb 13, 2010 11:02 pm
Posts: 420
Sorry here is the method with my reasoning. Hope it makes sense.

999 mum wrote:
Was the day of the week that he was born on given?

If so,
1. then the number of days can be calculated by 21*365+5.
(This gives the total number of days, +5 is to allow for the leap years with the extra days)

2. Divide the total by seven,
(as day 7,14,21 etc will always fall on the same day)
3. What ever the remainder is count on that many days of the week.
This is to work out the day of the week for the final incomplete week

I think this would work, but I have not worked it out or checked it. (Sorry)

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 Posted: Wed Jan 18, 2012 4:07 pm

Joined: Wed Apr 13, 2011 10:25 pm
Posts: 463
999 mum wrote:
Thingsbehindthesun wrote:
could you please explain why an additional 5 is added to number 1.

the extra day in leap years.

Thank you

_________________
;D

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 Posted: Wed Jan 18, 2012 7:46 pm

Joined: Thu May 26, 2011 8:37 pm
Posts: 577
Thanks for the reply. 999 mum you have no idea how much simpler this is than the methods online which are so very complicated that I gave up trying to teach them to my sons. That was exactly the formula I was looking for. Except... I don't know what the asterisk stands for. Is it multiply? Divide?

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 Posted: Wed Jan 18, 2012 8:13 pm

Joined: Sat Feb 13, 2010 11:02 pm
Posts: 420
menagerie wrote:
I don't know what the asterisk stands for. Is it multiply? Divide?

It is multiply. (sorry I have been using MS Excel for too long!)

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 Posted: Thu Jan 19, 2012 1:24 pm

Joined: Thu May 26, 2011 8:37 pm
Posts: 577
Thanks! I guessed it meant divide which would have messed up the calculations a bit!

This is great. I'll show the boys tonight and they can test it out.

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 Posted: Thu Jan 19, 2012 1:44 pm

Joined: Fri Nov 20, 2009 10:46 am
Posts: 425
Hi 999 Mum,

Just a quick question - so if they asked "which day of the week wuold be his/her 35th birthday" then instead of adding 5 would you add 8?

And if he/she was born on Monday and the remainder was 6 then would the birthday fall on Sunday.

Thanks!

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