20ml
To solve that, we need to use the dilution equation:
[tex]M_1V_1 = M_2V_2[/tex]
Volume, in this case, is measured in liters (L), thus it is necessary to convert from [tex]ml[/tex] to [tex]L[/tex].
And with that in mind, we can plug in the other variables into the equation. The first two cited on the questions are equivalent to [tex]M_1V_1[/tex] and the ones cited last are equivalent to [tex]M_2V_2[/tex], being, in this case, [tex]V_2[/tex] the value that we are looking for.
[tex]0.04 \times 0.1 = 0.2 \times V_2\\0.004 = 0.2 \times V_2\\\frac{0.004}{0.2} = V_2\\\boxed{0.02L = V_2}[/tex]
Remember, 0.02 is the value in liters (L), it is essential that you transform it back to the unit of the question (if requested).
If you are curious and would like to double-check the calculations, just eliminate other variables to test and proceed with the calculation:
[tex]M_1 \times 0.1 = 0.2 \times 0.02\\M_1 \times 0.1 = 0.04\\M_1 = \frac{0.04}{0.1}\\\boxed{M_1 = 0.04}[/tex]
Since the initial value for [tex]M_1[/tex] was 0.04L, we can now be sure that 0.02L is the correct value for [tex]V_2[/tex], the volume required to reach the equivalence point.