6. Container $A$ contained a mixture that is $40 \%$ acid, while container $B$ contained a mixture that is $60 \%$ acid. A chemist took some amount from each container, and mixed them. To produce 100 liters of mixture that is $17 \%$ acid, she needed to pour 70 liters of pure water to the mixture she got from containers $A$ and $B$. How many liters did she take from container $A$ ?
6. 5
Let $a$ be the amount (in liters) of mixture the chemist took from container $A$, and $b$ the amount she took from container $B$. Then $a+b+70=100$. On the other hand, computing the amount of acid involved in the mixtures, we have $0.40 a+0.60 b=0.17(100)$ or $4 a+6 b=170$. Solving for $a$ in the following system of equations:
$$
\left\{\begin{aligned}
a+b+70 & =100 \\
4 a+6 b & =170,
\end{aligned}\right.
$$
we get $a=5$.