You may also like

Triangle Incircle Iteration

Keep constructing triangles in the incircle of the previous triangle. What happens?

Vedic Sutra - All from 9 and Last from 10

Vedic Sutra is one of many ancient Indian sutras which involves a cross subtraction method. Can you give a good explanation of WHY it works?

Tournament Scheduling

Scheduling games is a little more challenging than one might desire. Here are some tournament formats that sport schedulers use.

Zeller's Birthday

Age 14 to 16
Challenge Level

What day of the week were you born on? Do you know?

Here's a way to find out.

For example, if your date of birth was 6 July 1989 :

$D = 6$ , $M = 7$ , and $Y = 1989$

If $M$ had been a 1 or 2, subtract 1 from $Y$ and add 12 to $M$.

$Y_F$ is made from the first two digits of $Y$ and $Y_L$ is made from the last two digits of $Y$.

To begin, work out the sum of all the integer parts of $2.6M - 5.39$ , of $\frac{Y_F}{4}$ , and of $\frac{Y_L}{4}$

Add $D$ and $Y_L$ into that total, and then subtract 2 lots of $Y_F$

Divide that final answer by 7 and notice the remainder .

A remainder value of 0 means the date was a Sunday, 1 means it was a Monday, 2 for a Tuesday, and so on.

Can you follow the method (what you actually have to do) ?

You could check some dates you happen to know the answer for ?

When you are getting good at using the method start to ask yourself how it works and why does it give the right result?

Why 2.6 and why 5.39?