Calculator pages are currently being checked and updated.
A series of pages is available from the top menu under Calculators.
Further calculators are available; they will all be available shortly on this blog.
In the meantime, these links will take you to the relevant pages.
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For a contrast, see the post No Calculators Allowed.
Estimation by rounding by cparkinson3 is is a complete lesson providing a great variety of problems from basic skills to more challenging ones where learners have to apply their understanding of estimation to solve real-world problems. All needed resources are provided.