Question 987448
Most acids would need to be measured in mass.  Percent concentrations could be mass/mass, mass/volume
or volume per volume.  Assuming your percents are as mass/volume:


Let L be the original volume of the original mixture and be the same as the final volume
after removing v and replacing with water.


{{{(0.42*L-0.42(20))/L=0.315}}}


Solve for L.