What is the total area of the red spots? The diagram involves a rectangle with dimensions of 10 and 20. Inside the rectangle one of its diagonals is connected. There are two circles of equal size that are tangent to each other and the rectangle. The video presents the solution to one version of the problem and then the solution to a second, much harder version, that was said to be assigned to 6th graders in China.

I read about this problem from Jin Yang on Twitter:   / 759079328965505025  

**Update 12 Aug: Many people suspected the second problem was too hard for 6th graders. It appears the first problem was assigned to 6th graders, and second problem (with the missing spot) was a misprint that went viral on social media. Jin Yang tweeted me with a link to the original problem assigned (in Chinese): http://wenku.baidu.com/view/de4510b07...

The Twitter thread from Jin Yang with the link to the original problem:   / 763406201468047360  

Blog post: http://wp.me/p6aMk4Ls

There are other solution methods described on Quora: https://www.quora.com/Whatistheare...

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