Answer
$12 \space hours$
Work Step by Step
Let the larger pipe fill the tank in $a$ hours and the smaller tank fill the tank in $3a$ hours.
(Fraction of the tank can fill by the larger pipe in a 1 hour)+ (Fraction of the tank can fill by the smaller pipe in a 1 hour)9=1
$\implies (\dfrac{1}{a}+\dfrac{1}{3a})9=1$
$\implies (\dfrac{9}{a}+\dfrac{9}{3a}) =1$
$\implies (\dfrac{9}{a}+\dfrac{3}{a}) =1$
$\implies \dfrac{12}{a}=1$
$a=12 \space hours$
