On a calendar put a box around four numbers.cal clip
Add the numbers in opposite corners.
For example 8+16 and 9+15.
What do you notice about the answers?

Put a box around another set of four numbers and try this again.
What happens and why?Will this always happen? Why or why not?

Try multiplying – perhaps you could use a calculator to help.

What happens in other months?

Find a random square with four dates as above on any calendar of any month of any year (all four dates must be within the same month). Add the numbers up and tell me the total. I can now guess what are your four numbers! Can you explain how and why this trick works? Try the trick on other people.

Do the same with a hundred square.

Again put a box around four numbers. Add the numbers in opposite corners.
What do you notice about the answers? Explain why this happens.

Try multiplying and explain what happens.

Click here to download the CALENDAR PATTERNS worksheet.

Teacher Notes CALENDAR PATTERNS

For a variation on this activity see NRICH

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