## Algebra 1

$12 \space hours$
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 $\implies (\dfrac{1}{a}+\dfrac{1}{3a})9=1$ $\implies (\dfrac{9}{a}+\dfrac{9}{3a}) =1$ $\implies (\dfrac{9}{a}+\dfrac{3}{a}) =1$ $\implies \dfrac{12}{a}=1$ $a=12 \space hours$