This comes from the "math missionary" Greg Tang whose website GregTangMath.com contains many math education games and resources. I saw it in a tweet by best-selling author Celeste Ng. The problem sparked arguments and discussion because adults wondered how it can be solved using mental math. Can you figure it out?

Sources
https://twitter.com/pronounced...._ing/status/93566193
https://gregtangmath.com/

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