







preuve par 9 
20010404 

Michel pose la question : Pourriezvous m'expliquer en détail comment fonctionne la preuve par 9 (pour une division et une multiplication). Je sais l'appliquer mais je ne sais pas pourquoi ça marche. Je ne retrouve pas la démonstration. Merci de m'aider. Exemple . 17x2=34 ; preuve par neuf : 1+7=8 ; 8x2= 16 ; 1+6=7 et 3+4=7, on peut donc supposer (sans affirmer) que cette multiplication a un résultat juste car la preuve par 9 est bonne, 7=7. Comment fonctionne cette preuve par 9??? Claude Tardif lui répond. 





The cube root of 729 
20141112 

Alexis pose la question : What would be the square root of 729 to the third power and could you
explain how to get the answer? Penny Nom lui répond. 





A magic/math trick 
20091204 

Valentin pose la question : What is the explanation for the following math trick: you think of any four digit number add those digits take that sum and subtract it from the first number then you say three of those new numbers in any order and the other person guesses the last digit.
How does he do it? Claude Tardif lui répond. 





The sum of digits of 4444^4444 
20090831 

SHIVDEEP pose la question : The sum of digits of 4444^4444 is A .The sum of digits of A is B .
Find the sum of digits of B ? Claude Tardif lui répond. 





A math trick 
20071210 

Megan pose la question : I need to write a mathematical explanation of why this works!
Start with a four digit number. (a positive integer, and all digits can NOT be the same. At least one must be different)
Rearrange that four digit number.
Subtract the smaller 4digit number from the larger.
Now circel one digit. (canNOT be zero, because that is already a circle)
Now rewrite that number excluding the circled digit.
Compute the sum of the digits.
Now write down the next multiple of 9 that is larger than the sum.
Subtract the Sum from the multiple. (multiple  sum of digits)
Report Difference = to number circled.
The resulting number should be the number that originally circled. Penny Nom lui répond. 





A number puzzle 
20060322 

A teacher pose la question : Students brought this website to my attention and asked why this puzzle worked... I'm not sure. The url is: http://digicc.com/fido/ and it tells you to choose a 3 or 4 digit random number with different digits. Write it down, rearrange, subtract the smaller from the larger. then circle a nonzero digit, type the remaining digits into the space provided and they will tell you the number you circled. Can you provide the reason that this works. Claude Tardif and Penny Nom lui répond. 





Back to the nines 
20060315 

Victoria pose la question :
Can you answer this problem, does an answer exist?
 Get a set of numbers 19 !
 Using the whole set of nine number tiles (digits 19), try to arrange them to make three 3digit numbers so that the sum of the first two is the third.
Can this be done without carrying over? If not can it be done without carrying over into the hundreds column? Claude Tardif lui répond. 





Pick a number greater than 1 
20040625 

A student pose la question : I understand that when you pick a number greater than 1 and less than 10; multiply it by 7 and add 23, then add the digits of that number until you get a one digit number. Then multiply that number by 9, add the digits of that number until you get a one digit number, subtract 3 from that number and divide the difference by 3; that this process will always give you the result of 2. Does this have a name or theory for it as to why the answer will always be 2? Penny Nom lui répond. 





What's it called? 
20040422 

Gerry pose la question :
I'm a father and a grandfather and have come up with a game for my offspring to play while we're on the road. When we see a license plate, the object is to be the first one to add all the numbers on it, and come up with THE one digit number that sums them up.
For example: ABC787 = 7+8+7 = 22 = 2+2 = 4
Another example is 2932 = 2+9+3+2 = 16 = 1+6 = 7
Up 'til now, I've called it just plain "Numerology", but I'm sure that there's a math term for what we're doing, and I'd sure appreciate it if you could tell me what it is!
Chris Fisher and Penny Nom lui répond. 





Divisibility by 9 
20001024 

Kelera pose la question : If the sum of the digits of a number is divisible by 9, then the number itself it divisible by 9. Why is that? How do you explain this? Penny Nom lui répond. 

