Sunday, August 14, 2011

ArithmeTRICK: Your Birth Day

I had bought a copy of Amazing Math With Your Kids at...I can't remember where. Skate Day probably. Anyway, it's full of games and activities to help with math concepts. We haven't used it much, but I picked it up today and this particular activity caught my eye. Bobby had said something the other day about a guy he knew who could figure out the day you were born in his head and I couldn't understand how. But now I know what to do:)

Here's the example they give:

1. Let's say your birthday is September 30, 1970. Let A be the year in which you were born.


2. Let B be the day of the year that you were born.

January, March, May, July, August, October, December = 31 days
February = 28 days (29 in leap year)
April, June, September, November = 30 days

So for September 30th, the answer is found like this:
31+28+31+30+31+30+31+31+30=273rd day of the year


3. Find C: C=(A-1)/4. Leave off the remainder.


4. Find D: D=A+B+C
D= 1970+273+492


5. For this step, do not use a calculator. Divide by 7. Notice the remainder.
2735/7 =390 r 5

6. Match the remainder to one of the days of the week below. That is the day you were born.

0 Friday
1 Saturday
2 Sunday
3 Monday
4 Tuesday
5 Wednesday
6 Thursday

So a person born on September 30, 1970 was born on Wednesday.

I checked this with my birthday and it's accurate.

What day were you born?


  1. Wednesday. I popped up the calendar on the computer and rolled the year back to 1974.

  2. I KNEW you were going to say something like that if you commented! LOL! Of course you can do it that way, but it's cool that it can be done using math:)

    I had actually used my cell phone to check mine. I was surprised that it went to 1981, but that's as far back as it went...30 years! AH!


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