Lời giải:
$\frac{M}{4}=\frac{\sqrt{x}}{\sqrt{x}+2}=1-\frac{2}{\sqrt{x}+2}$
$x\in\mathbb{N}; x< 101\Rightarrow x\leq 100$
$\Rightarrow \sqrt{x}\leq 10$
$\Rightarrow \sqrt{x}+2\leq 12$
$\Rightarrow \frac{2}{\sqrt{x}+2}\geq \frac{1}{6}$
$\Rightarrow \frac{M}{4}\leq \frac{5}{6}$
$\Rightarrow M\leq \frac{10}{3}$
Vậy $M_{\max}=\frac{10}{3}$ khi $x=100$