Question 28759
Let amount of 5% solution be x;.05x of this is phenob.

Amount of 25% must be 500-x; 0.25 of this is phenob.
EQUATION:
  Phenob + Phenob = 0.20(500)
0.05x+ 0.25(500-x)=100
5x+25(500-x)=10000
5x-25x=10000-12500
-20x=-2500
  x= 125 (millileters of 5% solution)
500-x=375 (millileters of 25% solution)
Cheers,
Stan H.