Multiplication Facts

Date: 02/12/2002 at 22:20:04
From: Levi Avalos
Subject: Multiplication 

My son brought home his homework, and there are two questions we don't 
understand:

1. Can you use the same multiplication fact to find 4x50 and 5x40? 
Explain. 

What is a multiplication fact?

2. Can you use the same multiplication fact to find 4x50 and 3x80? 
Explain. 

I don't understand what they are asking. Can you please help?

Thanks, 
Levi Avalos and his mother, Michelle

Date: 02/13/2002 at 22:15:29
From: Doctor Jodi
Subject: Re: Multiplication 

Hi Michelle and Levi!

These are some good questions.

First of all, a multiplication fact is part of the times table 
(like 2x3 = 6).

You know that 2x3 = 3x2, right? (In general, you can switch the order 
of multiplication.)

For your first question, I would think about 4x50 as 4 x 5 x 10 (since 
50 = 5x10)

And I would think about 5x40 as 5 x 4 x 10.

Which multiplication facts do both of these use? Can you come up with 
a way to do this multiplication in your head?

For your second question, we could think about 3x80 as 3 x 8 x 10.
Hmm. That doesn't look very much like what we had for 4 x 50 
(4 x 5 x 10). Can we make these look more alike?

A little bit:

We could write 8 as 4 x 2, so then we'd have 3 x 4 x 2 x 10 and 4 x 5 
x 10. These look a little similar - they both have a 4 and a 10 - but 
they're mostly different.

Can you figure out how to do 3 x 80 in your head? Rearranging the 
factors (the numbers in these multiplication facts) helps sometimes.

Have fun multiplying, and feel free to write back if you have more 
questions!

- Doctor Jodi, The Math Forum
  http://mathforum.org/dr.math/