Q&A #286

Teachers' Lounge Discussion: Teaching conversion of units of measurement

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From: Kari O'Connell

To: Teacher2Teacher Public Discussion
Date: 1999020219:40:31
Subject: unit conversions ideas

I am just starting a geometry unit on area and perimeter, and I insist
upon showing all units.  I found students often ask whether to
multiply or divide.  So now I tell them to ALWAYS multiply by "1", but
they have to figure out the correct form of "1".  For example, since 1
foot = 12 inches, I could write the number 1 as 12inches / 1 foot or I
could write it as  1 foot / 12 inches.  I choose, by picking the one
where the units I WANT are in the numerator.  That way the units I am
converting from, cancel out.   My success is based on the assumption
that in the previous algebra course, students have muliplied a x (b/a)

ex: convert 44 inches to feet:
     48 in x 1 ft       =  4 ft
             12 in 

 ex: convert 20 feet to inches:
 20 ft   x 12 in    =  240 ft
           1 ft 

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