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Lawrence Hall of Science
The Math Page: Skill in Arithmetic: Table of Contents
This website from The Math Page explains some helpful hints about how to do mental math.
Illustrative Mathematics
Illustrative Mathematics: 3.nbt, 4.nbt Rounding to the Nearest 100 and 1000
This task builds off of the skill of rounding to the nearest ten and hundred in third grade. Aligns with CCSS.Math.Content.3.NBT.A.1 Use place value understanding to round whole numbers to the nearest 10 or 100.
Lawrence Hall of Science
The Math Page: Skill in Arithmetic: The Meaning of Multiplication: Mental Calculation
If you are looking for a website that explains and defines the basics of mental multiplication with helpful hints then you have arrived at an excellent resource. There are even practice problems at the very end of the lesson.
Other
Tlsbooks.com: Printable Math Worksheets
You may print off addition, subtraction, multiplication, division, averages, fraction, and other miscellaneous worksheets that are related to mathematics. On the left hand side of this page you will also find links to other subjects,...