Question 159535
Let x=amount of water needed
Now we know that the amount of pure minoxidil in the 20ml of the 4% solution(0.04*20) plus the amount of pure minoxidil in the water (0) has to equal the amount of pure minoxidil in the final mixture(0.02*(20+x)).  So our equation to solve is:

0.04*20=0.02(20+x)  get rid of parens and simplify
0.8=0.4+0.02x  subtract 0.4 from each side
0.8-0.4=0.4-0.4+0.02x  collect like terms
0.4=0.02x  divide each side by 0.02
x=20 ml -----amount of water needed

Hope this helps---ptaylor