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You could try one of these wonders at Wonderopolis.
Khan Academy, grades K–12, free
Step-by-step videos and practice exercises make this a deserved staple in the academic website hall of fame. Every type of math is covered here in addition to computing, science, and history. Though it's best when kids already understand the concepts and just need help with the nitty-gritty, this resource gives kids access to independent, self-paced learning.
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