This Math Question From A Hong Kong Elementary School Test Has Adults Stumped

This math question, from an admissions test for an elementary school in Hong Kong, is going viral in China.

According to ChinaSmack, it was the second-most popular post on Chinese microblogging site Sina Weibo on June 5, when it was published.

The question was part of an admissions test for first graders. They had 20 seconds to answer. Can you solve it?

Stumped? Scroll down for the answer.

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Flip the image upside down, and you'll quickly see the solution. See readers' comments from the post on Sina Weibo translated into English at ChinaSmack.