Ask Dr. Math

Elementary Archive

Browse Elementary Number Sense/About Numbers
Stars indicate particularly interesting answers or good places to begin browsing.

Selected answers to common questions:
    About zero.
    Multiplying by zero.
    Casting out nines.
    Classifying numbers.
    Googol, googolplex.
    Number facts from the Web.

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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?

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 why.

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 words?

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?

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