Question 1166872
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We can overlook the fact that the people who write these problems don't know the science that says a 60% salt solution is not possible.  The problem is intended to give the student practice solving mixture problems.<br>
For a formal algebraic solution, we are mixing x liters of a 0% solution (distilled water) and (40-x) liters of a 60% solution to get 40 liters of a 45% solution:
{{{0(x)+.60(40-x) = .45(40)}}}<br>
Solve using basic algebra....<br>
If a formal algebraic solution is not required, here is a quick and easy way to solve any 2-part mixture problem like this.<br>
(1) You are starting with a 60% solution and adding a 0% solution, so you are heading towards a 0% solution; you are stopping when you get to a 45% solution.
(2) 45% is 1/4 of the way from 60% to 0%.  (Picture the numbers on a number line if it helps....)
(3) That means 1/4 of the 40 liter total is the 0% solution you are adding.<br>
ANSWER: Drain off 1/4 of the 40 liters, or 10 liters, and replace it with distilled water.<br>
CHECK:
.60(30) + 0(10) = 18
.45(40) = 18<br>