This question was used in China to identify gifted 5th graders. Supposedly, some of the smart students were able to solve this in less than one minute.
Answer: 9.
In order to solve this problem, you need to understand how the area of a parallelogram works. If you already know how the area of a parallelogram and the area of a triangle are related, then adding 79 and 10 and subsequently subtracting 72 and 8 to get 9 should make sense—but if you’re still confused, then check out this YouTube video for a more in-depth explanation.
