Q&A #770

least common multiple (LCM)

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From: Gail (for Teacher2Teacher Service)
Date: Nov 12, 1998 at 18:17:04
Subject: Re: least common multiple (LCM)

I have my students do lots of skip counting (which is, essentially, counting
by multiples). When they are familiar with that (but not before, because it
will mean nothing to them then), I list out what they are saying, and we
examine the information. I usually start with two numbers, like 3 and 6, or 4
and 12, and then move on to 3 and 4, etc.  Then I include a third number in
the mix. I want my students to notice from the lists that there are many (in
fact, countless) multiples that two numbers share. We make a connection
between "sharing" and "common". In fact, on the East coast we study colonial
times in the upper elem. grades, so I can even draw in a social studies
connection with the term, "common". Anyway, we identify all the numbers that
are common on the list, and soon it becomes evident there is a pattern
happening.  At that point, I like to ask them to find the smallest of all the
common numbers, or "the least common multiple".

Another way to do this is with a hundreds chart. Use colored pencils or
transparent colored chips.  Select a color for the first number, and color
all the skip-counted (multiples) numbers for that number. Then select
another number, and do that same thing using a new color. Anywhere two
colors end up on the same spot is a common multiple. The smallest one is the
LCM.  Hope your child finds this fun and meaningful.

 -Gail, for the Teacher2Teacher service

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