Question 263829
Let x=amount of 67% solution needed
Then 42-x=amount of 46% solution needed
Now we know that the pure herbicide in the 67% solution (0.67x) plus the amount of pure herbicide in the 46% solution (0.46(42-x)) has to equal the amount of pure herbicide in the final mixture (0.55*42).  So, our equation to solve is:

0.67x+0.46(42-x)=0.55*42  simplify
0.67x+19.32-0.46x=23.1  subtract 19.32 from each side
0.67x+19-32-19.32-0.46x=23.1-19.32  collect like terms
0.21x=3.78  divide each side by 0.21
x=18 liters------amount of 67% solution needed

Hope this helps---ptaylor