[tex]\displaystyle\bf\\A_{2n-3}^2=20\\\\(2n-3)(2n-4)=20\\\\(2n-3)~si~(2n-4)~~\textbf{sunt numere consecutive.}\\\textbf{Descompunem numarul 20 in produs de numere consecutive.}\\\\20 = 5\times4\\\\(2n-3)(2n-4)=5\times4\\\\\implies~~2n-3=5\implies2n=8\implies~\boxed{\bf~n=4}[/tex]
