Can you solve this tricky math problem using nothing but regular school math?

Classic brain training methods are perhaps puzzles like crosswords or sudoku, but in recent times, I have become more and more attracted to the type of challenge you’ll see below.

These types of puzzles have been flooding the web lately, probably because they are really fun!

These are old classic mathematical problems. When you were in middle or high school.

These tests are more fun when you find yourself trying to remember the math you learned as a child.

Can you find the correct number?

Okay, here comes the challenge.

In the picture below, you will face a math problem, and there are three different possible solutions you can pick from.