Question 47403
Let x = the number of ml of the 11% acid solution.  The chemist needs to add this to (700-x) ml of the 4% acid solution to get 700 ml of 6% acid solution.
Write the equation, but first change the percents to their decimal equivalents:

0.11x + 0.04(700-x) = 0.06(700) Simplify and solve for x.
0.11x + 28 - 0.04x = 42 Collect like-terms.
0.07x + 28 = 42 Subtract 28 from both sides of the equation.
0.07x = 14 Divide both sides by 0.07
x = 200
700-x = 500

The chemist needs to mix 200 ml of the 11% acid solution with 500 ml of the 4% acid solution to obtain 700 ml of 6% acid solution.