a) $f(1)=\dfrac{1}{1+1}=\dfrac{1}{2}=0,5$
$\to f(f(1))=f(0,5)=\dfrac{0,5}{1+0,5}=\dfrac{1}{3}$
$\to f(f(f(1)))=\dfrac{\dfrac{1}{3}}{\dfrac{1}{3}+1}=\dfrac{1}{4}$
b) $f(\sqrt{x})=\dfrac{\sqrt{x}}{\sqrt{x}+1}$
$\to \dfrac{\sqrt{x}}{\sqrt{x}+1}=\dfrac{5}{6}$
$\to 6\sqrt{x}=5\sqrt{x}+5$
$\to 6\sqrt{x}-5\sqrt{x}=5$
$\to \sqrt{x}=5$
$\to x=25$
