Question 4536
This question can not be solved with the given info.  You indicate you have a .2 or 20% solution of diazapam.  The problem does not say if this is .2 per gallon.  If you assume it is .2 per gallon it doesn't matter how many gallons you add you will never reach a .5 concentration.  If you are given a concentration of 20% solution of diazapam say, per 1/2 gallon, or pint or liter, or something along those lines you could solve this problem.