Question 44886
<pre><font size = 5><b>How many liters (L) of a 10% AgI solution 
must be mixed with 7 L of a 4% AgI solution 
to get a 6% AgI solution? 

Let X be the answer.  Then you can read your problem 
this way:

When X liters (L) of a 10% AgI solution are mixed 
together with 7 L of a 4% AgI solution, we get X+7 
liters of a 6% AgI solution?  What is X?

The equation comes from this reasoning:

When the 10% of X L that is pure AgI in the first 
stuff is added to the 4% of 7 L that is pure AgI 
in the second, we get the 6% of the resulting X+7 L 
which is pure AgI.    

So 

10% of X plus 4% of 7 equals 6% of X+7

or

0.10X + 0.04(7) = 0.06(X+7)

Solve that and you'll get X = 3.5 L 

Edwin</pre>