See also the
Dr. Math FAQ:
3D and higher
Browse Elementary Number Sense/About Numbers
Stars indicate particularly interesting answers or
good places to begin browsing.
Selected answers to common questions:
Multiplying by zero.
Casting out nines.
Number facts from the Web.
- Pentagon Puzzle [09/18/2002]
Write the numbers 1 through 10 so each side of the pentagon has a sum
- Percentage of Votes Cast [06/03/2003]
If you have two percentages, say 25% and 50%, can you do a percent
change between the two?
- Perfect Numbers [9/15/1996]
What is a "perfect number"?
- Pi's Last Digit [7/20/1996]
I know pi is a nonterminating decimal, but if there were a last digit
wouldn't the last digit be a 0?
- Placement of Commas in Writing Numbers [10/22/2003]
Why is the ones period named for the ones place value and why is the
thousands period named for the thousands place? Why is there not a
- Precise Numbers [10/18/2001]
A precise number occurs when its proper divisors multiplied together
equal the number: 6 is a precise number because 1*2*3 = 6. Is there
another name or formula?
- Prefixes in Math [08/21/2001]
What are the prefixes for 7, 8, 9, and 10?
- Prime and Composite Numbers, Sieve of Eratosthenes [01/28/1997]
I need a list of all prime numbers and all composite numbers up to 50.
- Primes and Repeating Unit Numbers [12/09/1998]
How do you prove this statement: For every prime number there exists a
repeated unit number that is a multiple of that prime.
- Problem Solving and Mental Process [10/03/2002]
Two numbers add up to 19. The difference between the two numbers is 3.
What are the numbers?
- Proof: 2 = 1? [10/03/1997]
a = b is factored to a^2 = ab by multiplying both sides by a...
- Proof that 1 + 1 = 1? [09/04/1997]
Can you shed any light on this subject?
- Properties, Defined and Illustrated [10/10/2001]
I need a page of information on the commutative, associative, and
- Rational Numbers and Square Roots [10/07/1998]
Our teacher told us to find all the rational numbers we could containing
a sqrt symbol...
- The Real Number System in a Venn Diagram [09/08/1998]
How do you construct a Venn diagram that illustrates the real number
- Reverse Polish Notation in Calculators [09/17/2005]
An introduction to "reverse Polish notation" and the terms "stack",
"push", and "pop" in that context.
- Roman Numerals [11/13/1997]
What are three ways Roman numerals are used today?
- Roman Numerals on Clocks [06/04/2001]
Is the Roman numeral on a clock written as IV, or IIII?
- Rounding to the Nearest Multiple of 5 [01/08/2005]
How do you round to the nearest 5? Would 27 round down to 25 while 28
would round up to 30?
- Rounding to the Nearest Tenth [8/29/1998]
Would 6.96 be 7.0 or 6.9?
- Saying Numbers Out Loud [03/06/2002]
I have recently been told that the use of "and" is incorrect. Is it "one
hundred one" or "one hundred and one"?
- Scientific Notation [09/16/1997]
I don't understand how you get the answer: 5x10 to the 3rd power = 5,000;
5x10 to the -3rd power = -5,000.
- Scientific Notation of Zero [09/09/2002]
What is the scientic notation of zero?
- Short Word Form [10/15/2002]
How is 474,136 written in short word form vs. standard form?
- Significant Digits [09/07/1998]
What are significant digits?
- Significant Digits and Irrational Numbers [09/24/2005]
How can you determine the signifigant digits of a non-terminating or
irrational number like pi?
- Significant Digits and Zero [05/19/1999]
When are zeros significant and when are they not significant?
- The Size of Infinity [09/03/1998]
What is infinity times 2? Can we measure infinity?
- Sizes of Infinities [01/31/1997]
How can you prove that one infinity is larger than another?
- Smallest 5-Digit Number [02/05/2002]
My dad says the smallest 5-digit number is 00000, but my teacher says it
is 10000. The dictionary says that zero is a digit. What's the right
- Spaces and the Number Line [02/26/2003]
If you say, 'How many degrees are there between -2 and 4?' would you
count the 0 degrees and answer 7 degrees or would you not count the
nought and answer 6 degrees?
- Square Numbers End in 0 1 4 5 6 9 [10/15/2002]
Why do all square numbers end in 0 1 4 5 6 9?
- Teaching Children Other Bases [02/20/2002]
Why teach children number systems other than the base 10 number system?
- Teaching Elementary Concepts [04/04/1997]
How can I teach rational counting, prime and composite numbers, and
square roots at a grade-school level?
- Three Consecutive Natural Numbers [09/17/2001]
Why is it that the product of any three consecutive natural numbers is
always divisible by six?
- Trick for Numbers Divisible by 3 or 9 [02/24/1998]
Proof of a trick for numbers that are divisible by 3 or 9.
- Trilogy, Tetralogy... [05/28/2003]
Why isn't there a numbering system for groups like trilogies?
- Use and Meaning of 'And' in Naming Decimals [12/01/2004]
Can you please tell me what the word "and" means in the word form of a
- Use of Plural with Decimal Numbers [02/27/2002]
What are the rules for 1.0001, 1.1, 1.5 or 1.999 ? Should we write
1.0001, 1.1, 1.5, 1.999 point or 1.0001, 1.1, 1.5, 1.999 points ?
Differences between plurals in English and French.
- Using Integers [01/05/1999]
I need some real-life examples of integers to show my students that they
need to be able to use them in math class.