Solution:

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 $0.40 a + 0.60 b = 0.17 \times 100$ or $4a + 6b = 170$. Solving for $a$ in the following system of equations:
$$\left\{\begin{aligned} a + b + 70 & = 100 \\ 4a + 6b & = 170 \end{aligned}\right.$$
we get $a = 5$.