The key to getting the correct answer - which is a third - is working out that the height of the triangle is two thirds the height of the square.
David explained: "Pink triangle and little triangle have all the same angles: they're 'similar'.
"Base of pink is twice base of little [triangle]. So height of pink is also twice height of little.
"So height of pink is 2/3 of whole square."
The final stage of solving the problem involves using the formula for working out the area of a triangle - which is half x base x height.
As the height is 2/3 of the whole square, you can take the base to be equal to 1 when using the formula.
Therefore, as David explains, the area is half x 1 x two thirds.
This means the area of the triangle a third of the area of the whole square.
Dozens of other social media users have been sharing their solutions to the trick puzzle online, with one explaining: 'The intersection height is the key'.
Another added: "My instinct says it's 1/3... but I'll have to work on the explanation!"
