已知 $3^{a}=2,3^{b}=\frac{1}{5}$, 则 $3^{2 a^{-} b}=$
$\because 3^{a}=2,3^{b}=\frac{1}{5}, \therefore a=\log _{3} 2, \quad b=\log _{3} \frac{1}{5}=-\log _{3} 5$,

$\therefore 2 a-b=2 \log _{3} 2+\log _{3} 5=\log _{3} 20, \quad \therefore 3^{2 a^{-} b}=20$.