Question 1158439
think of pure acid and add an unknown amount of water x
at the end there will be 5+x gallons at 10% or .5+.1x gallons of pure acid.  

start with 5 gallons of 0.18, or 0.9 gallons of pure acid=0.5+.1x
so 0.4 gallons=0.1x
x=4 gallons

0.9 gallons of pure acid has a 10% concentration. That would require 9 gallons of solution in all
since 0.9/9 is 10%
That would mean add 4 gallons of water


another way to look at it is that one is going from 18%/*5 to 10%*x and the new total is 9 gallons