Ask Dr. Math

Elementary Archive

Browse Elementary Multiplication
Stars indicate particularly interesting answers or good places to begin browsing.

Selected answers to common questions:
    Flashcards/worksheets on the Web.
    Learning to multiply.
    Least common multiple, Greatest common factor.
    Number sentences.
    Order of operations.

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Associative, Distributive Properties [9/5/1996]

Please explain the associative and distributive properties.

Basic Real Number Properties [07/31/1997]

I need information about basic real number properties: associative, commutative, closure, identity, inverse, distributive.

Cross-Multiplication [08/04/2002]

Can you explain cross-multiplication?

Distributive Property, Illustrated [09/28/2001]

How can I show (12*2) + (12*3) using the distributive property?

Finger Multiplication for the 9s [06/10/1998]

Do you have any tricks for learning the times table for 9?

Help With Multiplication and Percents [11/11/1996]

Can you help me with multiplication so that I can better understand percents?

Homeschooling Multiplication [7/7/1996]

My son can't seem to memorize his times tables and therefore can't divide.

Lattice Multiplication [8/30/1996]

Can you please explain the lattice method of multiplication?

Lattice Multiplication Explained [10/20/1999]

Can you explain why the lattice method of multiplication works?

Learning Only 36 of the Times Tables [11/04/2001]

My mother says that I only need to know 36 of the times tables because problems can be reversed to a higher problem. What are the 36 problems?

Least Common Multiple Puzzle [03/24/2003]

What is the smallest number divisible by 1,2,3,4,5,6,8,9,10 that is not 3600?

Manipulating Positive and Negative Numbers [8/9/1996]

Could you explain the rules for adding, subtracting, multiplying, and dividing positive and negative numbers?

Memorizing Multiplication Facts [02/25/2001]

I can't remember my multiplication facts. I have flash cards, but they don't work...

Memorizing Multiplication Tables [01/06/1997]

I am having a problem memorizing my times tables. How can I do this easily?

Multiples and Factors [06/02/2003]

What is the difference between multiples and factors?

Multiples of 9 Number Puzzle [03/23/2003]

If you take a two-digit number, add the digits, and subtract the sum from the original number, why is the answer always a multiple of 9?

Multiplying by 0 [03/09/2001]

Why, when multiplying any number by 0, do you get 0?

Multiplying by 9, Step by Step [11/05/2001]

I need help with 9 x 240,531.

Multiplying Two 2-Digit Numbers [03/25/1997]

How do you do 28 x 24?

Odd and Even Multiplication [02/17/1999]

Is it true that multiplying an even number and an odd or even number always gives an even number?

Order of Operations for Solving Equations [10/12/1995]

How would you multiply a question like this: -2(x-y)^2 ?

Order Of Operations in Four Steps [02/15/1998]

I need help figuring out what operation to do first.

Prime Numbers [01/31/1997]

What are the prime numbers and why are they prime numbers?

Properties of Real Numbers [05/20/1997]

What are the associative, the commutative, and the zero product properties?

Reciprocals [12/19/1996]

What is the use of reciprocals?

Rectangles and Factors [09/09/1998]

How many rectangles can you make with 10 small squares? 5 small squares? 12 small squares?

Remembering Multiplication Tables [2/5/1996]

Is there an easy way to remember multiplication tables?

Running Laps and LCMs [02/11/1999]

Bill, Bob, and John run 1/3rd, 1/5th, and 1/6th of a lap per minute, respectively. How many laps do they need to run to cross the finish line at the same time?

Russian Peasant Method of Multiplication [10/07/1998]

I understand the 'Russian peasant' method of multiplication, but not why it works.

Russian Peasant Multiplication [02/26/2001]

Can you explain why the Russian Peasant algorithm works? How do the numbers added relate to the multiplier and multiplicand?

Tips for Learning Multiplication Tables [08/13/1998]

I have great difficulty with my 4, 6, 7, 8, and 9 times tables...

Using Parentheses [11/16/1998]

Can you explain how to use parentheses to tell the order of operations?

What is 4/5 x 10? [2/10/1995]

What is the answer to this fraction? Please explain.

What is an exponent? [8/8/1996]

Can you explain what an exponent is?

Working with Integers [01/12/1999]

Could you explain how to divide, multiply, add, and subtract integers?

3/4 of _____ Is 90 in Three Ways [03/24/2010]

In a multiplication problem like "Three-quarters of _____ is ninety," how do you fill-in- the-blank with the factor that satisfies the equality? Doctor Ian presents three different methods: guess-and-improve; the equivalency between multiplication and division; and multiplying through by a reciprocal.

Adding a Zero When Multiplying a Decimal [06/21/2001]

I don't understand why you add a zero when multiplying decimals.

Adding Tax and Tip [07/19/1998]

In pricing our hotel luncheons, we charge a base price plus a 6.8% tax and a 20% service charge. How would I find the total in one step?

Additive Identity and Other Properties [01/03/1999]

Can you explain the identity of zero and the commutative and multiplicative properties? Why are they special?

Algebraic Explanation for Wrist Multiplication [09/10/2004]

The trick behind the technique of wrist-multiplying is explained by use of a standard algebraic multiplication formula.

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