Question 77099
Let x=amount of 4% solution
Then 40-x=amount of 20% solution
 Now we know that the amount of pure medicine in the 4% solution (0.04x) plus the amount of pure medicine in the 20% solution 0.20(40-x) must equal the amount of pure medicine in the final solution 0.16(40).  So, our equation to solve is:

0.04x+0.20(40-x)=0.16(40) get rid of parens

0.04x+8-0.20x=6.4  subtract 8 from both sides
0.04x+8-8-0.20x=6.4-8  collect like terms

-0.16x=-1.6  divide both sides by -0.16
x=10 ml--------------------------------------amount of 4% solution

40-x=40-10=30 ml-----------------------------amount of 20% solution

CK

0.04(10)+0.20(30)=0.16(40)
0.4+6-6.4
6.4=6.4


Hope this helps-----ptaylor