[johnisgood]

Very easy to solve, just like it is easy to solve many other ones once you know the tricks.

I recommend this book: https://www.amazon.com/Secrets-Mental-Math-Mathemagicians-Ca...

[jabbywocker]

Completely missing the point on purpose?

[johnisgood]

Elaborate.

[jabbywocker]

you don’t solve it by brute forcing possible solutions until one sticks

[johnisgood]

Yeah, read it in another comment. Why do you think doing calculations in your head is brute-forcing? Many people can do it flawlessly, without even knowing of these "tricks". They just know. Is that brute-force?