Calculator Tricks 4 By Brian Carruthers Please see

Calculator Tricks 4 By Brian Carruthers (Please see notes on slide 1)

Lucky 7 or Unlucky 13

Step One Think of a three-digit number n Enter it twice on your calculator. n For example: 123123 n Now concentrate on the display. n

Step Two I think that your number is divisible by 11. n Do you believe me? n If you don’t then I’ll prove it to you. n

Use your calculator to check that your number is divisible by 11. Am I right?

Step Three But, guess what: n Your answer displayed on your calculator is divisible by 13. n Again, use your calculator to check the answer. n

Now, divide your result by your original three-digit number. n Your final answer will be : n 7

Watch this: Enter your original three-digit number twice on your calculator. n Divide that number by 11. n Now divide your result by 7. n Divide your answer by your original three digit number. n

The answer will be 13

Why does this work? n n Entering a three-digit number twice For example: 123123 is the same as multiplying it by 1001. 123 x 1001 = 123, 123. Because 1001 = 7 x 11 x 13, the six-digit number will be divisible by 7, 11, 13, and your original threedigit number.

Watch this: 345345 divided by 11 = 31395 divided by 13 = 2415 divided by 345 = 7 678678 divided by 11 = 61698 divided by 7 = 8814 divided by 678 = 13.

Now what did you think of that?