A solution containing 30 percent juice is mixed with a solution containing 10 percent juice to make 100 gallons of a 12 percent juice mixture. how much of the 30 percent juice solution was used

let there be x gallons of 30 percent juice, then there are (100-x) gallons of 12 percent juice.

0.30x+0.10(100-x)=100*0.12
0.3x+10-0.1x=12
              0.2x=2
                   x=10

10 gallons of 30 percent juice should be used.