Question 24767
Let x represent the amount needed.

Since 6 gallons is already added to 4% = 4(6)
Need to know how much of 3% of 6 gallon =(x+6)---> amount of gallon(s) needed
----------------------------------------3(x+6)---> Amount of 3% of that gallon needed


Expression: 
4(6)=3(x+6)
24=3x+18
3x=6
{{{3x/3=6/3}}}
x=2


Hence about 2 gallons are needed for 30%.
Paul.