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.
- 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.
- What is 0? [5/21/1996]
Is 0 a positive or a negative integer, or neither?
- What is a Number Base? [04/19/2003]
The base eight number 7n3 is equal to the base 7 number 1n35.
What digit does n stand for?
- What is a Parenthesis? [07/17/2000]
What does the mathematical term "parenthesis" mean?
- What is Expanded Notation? [08/22/2001]
My daughter was asked to rewrite each number as expanded notation. What
is expanded notation?
- What is Number Sense and How Can You Improve It? [08/29/2007]
What does the term 'number sense' refer to on standardized math test
results? Is there any way to increase your number sense?
- When in Rome, Know Your Place — Less a Written Notation for It [01/27/2015]
An adult wonders how Romans could have developed their rules for subtraction
absent place value. Citing their use of the abacus, Doctor Peterson distinguishes
between operating with the concept and representing it.
- Where Does Pi Come From? [11/04/1996]
If pi is a ratio of lengths that can be measured only so accurately, how
can we know the value of pi so exactly?
- Who Invented Integers? [11/29/2001]
And when were integers invented?
- Whole Numbers and Multiples [09/11/2001]
My students and I have been trying to figure out what numbers are in the
set of "Whole Numbers." Is zero a whole number?
- Why Casting Out Nines Works [10/13/2008]
Can you explain in simple terms why the Casting Out Nines method works?
- Why Commas after Every Three Places? [07/10/2001]
Why do we put commas after every three places in a number like 200,000?
- Why Estimate? [08/16/2002]
Estimate to the nearest thousand a solution to Dave's subtraction
problem. The numbers are: 6,107 - 2,980 = 4,127.
- Why Rules? [08/15/2001]
Why do we need to have rules for order of operations?
- Why Study Prime and Composite Numbers? [01/25/2001]
My husband and I wonder why this is taught in school at all. Who uses
this in the real world? Why does someone need to know whether a number is
a prime number or not?
- Why Two Odds Always Sum to an Even [03/21/2002]
Why do two odd numbers always equal an even number but two even numbers
never equal an odd number?
- Why Zero is neither Positive nor Negative [10/5/1995]
My class and I are wondering if 0 is a negative or a positive number and
- Word for a 225th Birthday [07/10/2001]
This year America is celebrating its 225th birthday. As we celebrated the
bicentennial in 1976, is there a term for 225 years?
- Working in Base 7 [08/02/2007]
How do I add, subtract, multiply and divide in base 7?
- Writing Numbers [11/24/2002]
Which words are okay to capitalize and when are hyphens used between
- Writing Numbers in Words [09/15/1997]
Write each number in words: 1) 73.78007 2) 2.900087 (3) 34.4939.
- Zero and Negative Infinity [06/02/2003]
What is the smallest number? Zero or negative infinity? Explaining
concepts to a four-year-old.
- Zero Before the Decimal Point? [10/06/2000]
What is the merit of writing the zero before the decimal point in numbers
less than one, for example 0.25?
- Zero for Children [03/16/2002]
When you are teaching children to count (1-10), do you include zero? Why
or why not? How do you explain to a child (K-2) what the value of zero
is, and is zero just a place holder?
- Zero in the Denominator [8/14/1996]
What is the value of 4/0 * 0/4?