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.
- 1 and 0: Prime or Composite? [10/06/1997]
Is the number one a prime or a composite number? Why? What about zero?
- Adding and Subtracting Roman Numerals [10/07/1997]
Do you have any suggestions for how to teach adding and subtracting of
- Adding in Base 9 and Base 5 [10/21/1997]
In 3rd grade my son is learning how to add in bases other than 10.
- Ancient Math Symbols [09/07/1997]
I need to know the numerals for 1,10 100, 1000 in Arab, Samarian, Greek,
Roman, and Hindu.
- Base 2 Subtraction and Clock Arithmetic [01/26/2001]
We know that computers use the "2's complement" method to do subtraction
of base 2 numbers, and that this method works every time, but we have no
idea why it works.
- Base Number [10/07/1998]
What does base number mean?
- Composite Numbers [1/30/1996]
What are Composite Numbers?
- Converting Fractions to Decimals [01/08/1997]
How do you convert fractions to decimals? How do you turn decimals into
- Difference Between Zero and Nothing [12/12/1996]
What is the difference between zero and nothing?
- Explanation of Place Values [05/08/2000]
What is meant when we say that as you progress from the units place to
the tens place to the hundreds place, the value is increasing ten times
the number on its right?
- Learning Addition and Multiplication with Algebra [3/16/1996]
How do you know what the (a) is in algebra? I saw this on a picture:
a(1/a) = 1/a(a) = 1...
- Multiplying by 0 [03/09/2001]
Why, when multiplying any number by 0, do you get 0?
- Numbers between 0 and 9999 [7/15/1996]
How many whole numbers are there between 0 and 9999?
- Numbers: Cardinal, Ordinal, Nominal? [10/25/1999]
Which group of numbers - cardinal, ordinal or nominal - does "time" fit
into? Are there other types of numbers as well?
- Perfect Numbers [08/14/1997]
What is the highest perfect number that has been found? How many perfect
numbers are there? What are they?
- Perfect Numbers - Basics, History [11/3/1996]
What is the next perfect number after 28?
- Pi in Real Life [7/1/1996]
We have just covered circumference of circles and pi. The children asked
how this concept is used in real life and who pi is named after.
- Pi, Irrational Numbers [12/29/1994]
What is Pi and how does it work? What are rational numbers and what are
irrational numbers? How do they work?
- Plotting Coordinates on a Graph [07/02/1999]
How can I solve a problem like 3+(-2) = ? using a number line, and how
can I plot coordinates like (-4,3) on an X-Y graph?
- Prime Numbers vs. Prime Factors [01/13/1997]
What is the difference between a prime number and a prime factor?
- Properties of Real Numbers [05/20/1997]
What are the associative, the commutative, and the zero product
- Rational and Irrational Numbers [2/2/1996]
What is the difference between rational and irrational numbers?
- Real and Other Numbers [08/29/2001]
What is a real number? What are the different kinds of real numbers? Are
there any more numbers besides real numbers?
- Repeating Decimals [5/31/1996]
I claim that 0.9 repeating is equal to 1...
- Rounding to Specific Number of Significant Figures [05/30/2003]
How do I round to five significant figures?
- Rules for Significant Figures and Decimal Places [03/23/1999]
Accurate measurement: What are significant digits? When do we use them?
- Russian Peasant Method of Multiplication [10/07/1998]
I understand the 'Russian peasant' method of multiplication, but not why
- Scientific Notation: Exponents [8/29/1996]
What do you call 0.000000000000000000000002?
- Signs: Rules, Number Line [07/13/2001]
What is (-) - (+) = ? or (+) + (-) = ?
- Subdividing Numbers, Number Line [12/20/1994]
I know about negative numbers and positive numbers but are there any
other numbers? What is a number line?
- Triangular Numbers [11/15/1995]
Second and third graders ask why 1 is a triangular number.
- Using Parentheses [11/16/1998]
Can you explain how to use parentheses to tell the order of operations?
- Venn Diagram of Our Number System [12/13/2002]
I don't know how to include complex numbers that consist of a real
part and an imaginary part. Can you please diagram this for me?
- What is i? [12/15/1995]
I heard about i being an imaginary number. Can you please tell me what i
- What is Log? [2/26/1995]
What is 4 to the half power, and what is log?
- Why does PEMDAS work? [7/9/1996]
Why does the PEMDAS (Parentheses, Exponents, Multiplication, Division,
Addition, Subtraction) order of operations really work?
- Why Do We Round? [12/18/2002]
What's the whole point of rounding?
- Working with Integers [01/12/1999]
Could you explain how to divide, multiply, add, and subtract integers?
- Writing Commas in Numbers [07/11/2003]
Is it true that in France where we in America use decimals in math,
they use commas, and where we use commas they use decimals?
- 0.999... and Infinity [09/17/2003]
How does x = 0.9999... forever turn into x = 1?