Math How To..

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Decimals: Place Value

http://www.enchantedlearning.com/math/decimals/placevalue/ Advertisement.

Pre-Algebra - Math Learning Guides

http://www.shmoop.com/pre-algebra/ Basic operations are the building blocks and rules of math.

Rounding Numbers: EnchantedLearning

http://www.enchantedlearning.com/math/rounding/ When a number is rounded (or rounded off), it is approximated by eliminating the least significant digits. When rounding, you are finding the closest multiple of ten (or one hundred, or other place value) to your number.
http://www.murderousmaths.co.uk/games/primcal.htm Before we start with the fun (which involves a trick and a special prime number calculator ), there's just one thing you need to know:

Murderous Maths: Prime numbers!