Elementary Math Fun: Wow Your Friends with This Numbers Trick

If you are looking for a fun way to interest your student or child in basic math, teach him the following numbers trick. Once he has mastered it he's sure to get considerable enjoyment wowing his friends and relatives with his mathematical skill.

Step One: Give your friend a piece of paper and ask him to write down a number from 1 to 10. Don't let him show you the number or to see the paper as he does the following basic math calculations.

Step Two: Ask your friend to multiply the number by 5.

Step Three: Ask your friend to add 6 to the product from Step Two.

Step Four: Ask your friend to multiply the result from Step Three by 4.

Step Five: Ask your friend to add 9 to the product from Step Four.

Step Six: Ask your friend to multiply this last sum from Step Five by 5.

Step Seven: Ask your friend to tell you the final result of his calculations (but without showing you his paper), pretend to ponder deeply, then astound him by naming the original number.

Ah, but how does one figure out the original number from the final answer provided in Step Six? Here's the last steps of the numbers trick for the "master" mathematician to perform his magic.

Step Eight: The last two digits in the final answer will always be 65. Drop these two numerals and look at the remaining digits.

Step Nine: Subtract 1 from this number and you will have the original number your friend selected.

Let's look at an example. Say your friend selected the number 2. Here are each of the numbers trick's steps: 2*5 = 10; 10+6 = 16; 16*4 = 64; 64 + 9 = 73; 73*5 = 365

Your friend would have said "365"; you would have dropped the 65 and ended up with 3. Subtract 1 from the 3 and you have 2 as your original number.

For elementary math teachers:
I can see how you could easily use the above numbers trick as a fun math lesson plan. Ask each child to do the following steps (without letting you see their work, obviously), then amaze them with your mathematical skill as you pretend to read their minds by correctly guessing their original numbers. Afterwards teach them the final steps of the numbers trick so that they can go home and amaze their families with their mathematical genius.

Blessings!

Source
Editors. The Book of Knowledge. The Children's Encyclopedia. Volume 16