Question 730234
These problems are based on two simple equations.  1.  The concentration of a solution is (Amount of Important Stuff)/(Total Amount of Stuff).  In this case the important stuff is water.  2.  The amount of stuff in a solution is percent/100 * total amount of solution.<P>
The initial solution is 400ml and 25%, so there is 25/100 * 400 = 100 ml water.<P>
The amount of water added is unknown, so call it x.  That x adds to both the amount of water, and to the overall amount of solution. <P>
The new concentration is (100+x)/(400+x) = 32/100 = .32.  Solve for x.<P>
100+x = .32x + 128<P>
.68x = 28<P>
x= approximately 41.18 ml water