Question 709269
The water to add is 0% acid.  This means it is part of the volume change but not part of the quantity of acid change.  ONE variable.


let v be the volume of water to add to the 20% acid.  Result of the mixture must be 5% acid.


{{{15*20/(v+15)=5}}}

{{{5(v+15)=15*20}}}
5v+5*15=15*20
5v=15*20-5*15
5v=15(20-5)
5v=15*15
{{{highlight(v=45)}}}, ml. of water to add.