The whole Let's Investigate series (including books on angles, numbers, triangles, quadrilaterals, circles, time, rate, statistics, and others) are very well put together. They appeal to upper elementary through junior high ages. Interesting diagrams and pictures, along with succinct narrative. I've given these books to students to peruse while fellow students were working. The books effectively engaged them. They can also be used well for whole class extension activities. Good gift for a kid who has a math bent, or possibly for one whom the schools have left behind.
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