The Jinx & Birthday Puzzles
Student’s page
Here is a puzzle that will always jinx you. Follow these steps.

Pick a number ______
Add 11______
multiply by 6 ______
subtract 3 ______
divide by 3 ______
Add 5 ______
Divide by 2 ______
Subtract the original number ______

  • Try it with three different numbers.
  • Do you think it will always work? Why do you think that?
  • To see if it always works, make a spreadsheet model of this puzzle.

Here’s how:

  • Open Appleworks (or Excel)
  • Type in the text as you see it above.
  • Enter the formula =D3 + 11 in cell D4
  • Enter the formula =D4 * 6 in cell D5.
  • Continue until you have entered the formulas for cells D6 through D10.
  • After you finish entering the formulas, enter a numeric value in D3, like 6.
  • Look at cell D10. Does it show 13? (If not, check your formula in D10.)
  • Did D10 show 13? Try a different number in D3. What happens?
    • Did you get 13 in cell D10? Well, that means your number (6 in this case) jinxed you. You got 13 – the unlucky number!
    • Make 3 additional columns.
    • Highlight D3 through G10. (It forms a rectangle.)
    • Choose Fill Right from the Calculate menu.
    • Next enter different numbers in D3, E3, F3, G3 so it looks something like this. Try your own numbers.

    Can you find a number that will not always end up as 13?

    If not, why not?

    Puzzle #2: The Birthday Puzzle
    Create a Birthday puzzle spreadsheet for the puzzle below. See if you can figure out what has to be done in order to figure out how this “mind reading” trick works.
    • Ask the participants to write down the number of the month of the year in which they were born.
    • Write the number of the day they were born.
    • Have them do the calculations listed below.
    • Ask a participant what number they got. From that number the Math Magician hears, he or she can determine the personís birthday. See if you can figure out what he does.
    Here is the information the “magician” asks for
      • Month of Birthday
      • The number of the day of birthday
      • Multiply month by 5
      • Add 7
      • Multiply by 4
      • Add 13
      • Multiply by 5
      • Add the day of the month
    Build the spreadsheet. Collect some data. From the data see if you can figure out whatís going on.. Add one more row to the spreadsheet called “the secret formula” so that the birthday is revealed.  

    Hint: If your birthday is on November 5th the number you should get from the spreadsheet is 1310. How can you find the date of your birthday from that number? (Hint Your birthday is 1105 so what if you subtracted 205 from 1310?)

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