Wolfram Research
Wolfram Math World: Significant Digits
This site defines and describes significant digits along with examples. Includes references, citations, and links to further information.
Other
Dan's Math: Basic Skills Topics
College professor Dan Bach offers a review of various basic topics in arithmetic, pre-algebra, and algebra. Order of operations, exponents, square roots, variables, and combining terms are just a few of the many topics covered.
Other
Easyphysics: Chapter 1 Basic Skills
The resource examines the basic math skills needed for studying physics. Some topics include units, scientific notation, significant digits, and trigonometry. Example problems, graphs, and a chapter quiz are included.
Math Is Fun
Math Is Fun: Rounding Numbers
What is rounding, and what does it mean to round up and round down? Learn how to round numbers via simple steps and examples.
Math Is Fun
Math Is Fun: Approximate Solutions
Sometimes it is difficult to solve an equation exactly. But an approximate answer may be good enough!