Posted by Fai on Saturday, December 28, 2013 at 11:48am.
you need to use 3 solutions of drug H containing respectively 50%, 20% and 5% to make 600ml of 10% drug H. How much of the 50% will you use?

no, math  Damon, Saturday, December 28, 2013 at 12:34pm
try 400 ml of 5% and 200 ml of 20% for example then you need NONE of the 50%.
Unless you have another criterion, the problem has many solutions. 
steve for your solution  Steve, Sunday, December 29, 2013 at 6:23pm
If the amounts of the three solutions are x,y,z, respectively, then all you have given is
x+y+z = 600
.50x + .20y + .05z = .10*600
There are many solutions (as it were), such as
5% 20% 50%
400 200 0
440 140 60
500 50 50
520 20 60
533 0 67