Here's a fun shortcut solution to weighted average questions:
Consider the distances on the number line between each of the percentage solutions and the desired solution:
They are 30% and 5% away respectively, and these distances are in a ratio of 6:1. That is precisely the ratio of solutions needed.
Thus, we can divide 735 liters by 7 to get 105 liters, which volume represents 1 part in the above ratio.
So she should mix 105 liters of 30% solution with 630 liters (105 x 6) of 65% solution.
A more traditional approach would have you setting up a system of linear equations in 2 variables as follows:
x: volume of 30% soln needed
y: volume of 65% soln needed
x + y = 735
.3x + .65y = .6 x 735
