Mentally add 10 or 100 to a given number 100–900, and mentally subtract 10 or 100 from a given number 100–900.

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Ages: 9 - 18
Arthur Benjamin Does "Mathemagic"
From YouTube, produced by Ted TV
In a lively show, mathematician and magician Arthur Benjamin races a team of calculators to figure out 3-digit squares, solves another massive mental equation and guesses a few birthdays. How does he do it? He’ll tell you.   (15:14)
Found by laneyk in 2.NBT.B.8
April 14, 2010 at 08:46 PM
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Not Right For WatchKnowLearn
Ages: 6 - 8
Math Counting Song- Adding 5 + 5 (Pennies in my Pocket)
From YouTube, produced by Mr. R.
Mr. R.'s math song about addition fact 5 + 5. (02:29)
Found by raknyc in 2.NBT.B.8
May 19, 2010 at 08:01 AM
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Not Right For WatchKnowLearn
Ages: 7 - 12
How to Do Math In Your Head : Fast Multiplication Techniques
From YouTube, produced by Expert Village
Learn mental math multiplication tricks  Learn to multiply large numbers in your head.  Uses benchmark numbers. Suggests practicing mental math to get better at it.  Captions appear on the screen. (02:46)
Found by ronna_37 in 2.NBT.B.8
December 8, 2009 at 10:29 PM
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