NUMBER TRICK This is for secondary students and top primary. On any calendar of any month of any year put a box around four dates, two in one week and two in the following week (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.
For younger learners, use a calendar for the New Year. Ask about their birthdays. Find their birthdays on the calendar. Talk about the weeks, count the 7 the days of the week and talk about their names. On what days of the week will their birthdays occur in this year? Why do their birthdays occur on different days of the week in different years? Talk about the months, count the 12 months of the year and talk about the names of the months. In what months will their birthdays occur this year? How many days to their next birthday? How many weeks is that? How many months?
On the calendar put a box around four numbers.
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?
Try multiplying and explain what happens.
For a variation on this activity see NRICH