







The cube root of 729 
20141112 

From Alexis: What would be the square root of 729 to the third power and could you
explain how to get the answer? Answered by Penny Nom. 





Six nines 
20100916 

From Steph: It's sort of one question. We have to use six nines to get the numbers 130. I got all but 22, 23, 24, and 30. Like 15 is 9+9(9+9+9)/9. Answered by Penny Nom. 





A magic/math trick 
20091204 

From Valentin: 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? Answered by Claude Tardif. 





The sum of digits of 4444^4444 
20090831 

From SHIVDEEP: The sum of digits of 4444^4444 is A .The sum of digits of A is B .
Find the sum of digits of B ? Answered by Claude Tardif. 





Four nines to make 100 
20081127 

From jane: use 4 nines in a math equation that will give you an answer of 100 Answered by Penny. 





A math trick 
20071210 

From Megan: 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. Answered by Penny Nom. 





Four 9s 
20061215 

From A student: Can you make 67 with four 9's without using the ceilling method? Answered by Stephen La Rocque. 





A number puzzle 
20060322 

From A teacher: 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. Answered by Claude Tardif and Penny Nom. 





Back to the nines 
20060315 

From Victoria:
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? Answered by Claude Tardif. 





Pick a number greater than 1 
20040625 

From A student: 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? Answered by Penny Nom. 





What's it called? 
20040422 

From Gerry:
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!
Answered by Chris Fisher and Penny Nom. 





X.9999... and X+1 
20030823 

From David: I have read your answers to the questions on rational numbers, esp. 6.9999... = ? and still have a question: The simple algebraic stunt of converting repeating decimals to rational numbers seems to work for all numbers except X.999999.... where X is any integer. The fact that the method yields the integer X+1 in each case seems to violate the completeness axiom of the real numbers, namely that there is no space on the number line which does not have an number and conversely that every geometric point on the number line is associated with a unique real number. In the case of 3.999... for example, it seems that both the number 4 and the number 3.9999.... occupy the same point on the number line. How is this possible??? Answered by Penny Nom. 





100 from four 9's 
20020327 

From A student: My teacher gave us a math problem to try and figure out and I am stumped. The problem was this.... by only using four nines, how can you get an answer of 100? You can add, subtract, multiply or divide...anything you need to do ,but you can only use four nines. Answered by Leeanne Boehm. 





How do you get to 100 by using 6 nines? 
20011117 

From A student: How do you get to 100 by using 6 nines? Answered by Claude Tardif. 





Six nines 
20011009 

From A mom: My middle schooler (sixth) has to calculate the integers 020 using only 6 nines. We have done all but the integer 14. He can not use decimals or double the nine like 99 or 19. the fraction 9/9 is okay. Keep in mind of course the order of operations. Answered by Claude Tardif. 





Divisibility by 9 
20001024 

From Kelera: 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? Answered by Penny Nom. 





Nines and ones 
19991105 

From Greg Miller: Using the numbers 1, 1, 9, 9, only once each, how can I create an expression that equals 10 using only the basic functions of addition, subtraction, multiplication, division, and/or parentheses? Answered by Claude Tardif. 





A Place Value Curiosity 
19980525 

From Ed: I was visiting with an elderly gentleman this afternoon. He showed me this curiosity and then asked if I could explain it to him. Can you provide an explanation of why the 9 or multiple of 9 keeps occurring in this procedure? Choose any number, say 125 and add the digits to get 8. subtract the 8 from the 125 and the result is 117. Add the digits in 117 to get 9. Subtract the 9 from the 117 to get 108. Add the digits in 108 to get 9. If this procedure continues a 9 or a multiple of 9 reoccurs. What is the mathematical explanation behind this happening? Answered by Denis Hanson. 





preuve par 9 
20010404 

From Michel: 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??? Answered by Claude Tardif. 

