Another Just-For-Fun Topic

impressive solution!

I tried a direct attack with the symbolics and it fails ("no solution found") but at least it could verify the solution you found.

So I guess a brute force approach using a loop testing all multiples of 3 evaluated symbolically may come up with the solution - but probably not in a lifetime.

Apropos time - the result is the same in real Mathcad (V.15) but the difference in calculation time compared to Prime is quite remarkable!

Prime 11:

Mathcad 15:

But of course, symbolics makes it easier to arrive at the solution using your thoughts: