Question 804046
In this question, we can see that we have two types of solutions here: the 10 % solution and the 60% solution and that altogether, we need 200 liters of a solution that is 50 % acid.


x stands for the 10 % solution and y stands for the 60 % solution. We need 200 liters of the solution. Hence:

x + y = 200  (a)

The next equation would be what is needed to make 200 liters that is 50% acid.
Hence:

0.10x+0.60y = 0.50 * 200
0.10x+0.60y = 100         
Multiply by 10 to get rid of decimals: x+ 6y = 1000  (b)

Multiply (a) by -1 and then we can add equations (a) and (b)

-x-y = -200  (a)
x+6y =  1000 (b)

5y  = 800
 y   = 160

This will give us  x+y = 200
                   x   = 200- 160
                   x   = 40