Proving $\sqrt{x+y-x^2}+\sqrt{y+z-y^2}+\sqrt{z+w-z^2}+\sqrt{w+x-w^2}\le4\sqrt2-3$ I found this  inequality using unusual calculations in maths Olympics
  and I wonder if some clever teenager could prove it using their elementary knowledge of mathematics.
Let $x,y,z,w$ be non-negative numbers such that $$x+y+z+w=1$$ Prove that
$$\sqrt{x+y-x^2}+\sqrt{y+z-y^2}+\sqrt{z+w-z^2}+\sqrt{w+x-w^2}\le4\sqrt2-3$$

It depends on what you consider to be elementary.
By cauchy-schwarz:
$$ \sum\limits_{cyc} \sqrt{x+y-x^2} \leq2\sqrt{\sum\limits_{cyc}x+y-x^2}=2\sqrt{\frac{7}{4}+\frac{1}{4}(x+y+w+z)^2-(x^2+y^2+w^2+z^2)}.$$
Now, by jenson we have that $$(x+y+w+z)^2\leq 4(x^2+y^2+z^2+w^2). $$ 
Hence we conclude that $$ \sum\limits_{cyc} \sqrt{x+y-x^2} \leq \sqrt{7}.$$ 
Edit: I would consider this elementary since jensen and cauchy-schwarz are some of the first inequalities one learns after AM-GM. Moreover, we have equality in the above if and only if$x=y=w=z=1/4.$
