SOLUTION: How much pure disinfectant must be added to 30 gallons of an 8% solution to increase its strength of 25%?

Let x be the volume of the pure disinfectant under the question, in gallons.

Then the volume of the new mixture will be (30 + x) and the volume content of the pure disinfectant in the new mixture 
     will be 0.08*30 + x gallons.


So, the concentration equation is

 = 0.25.

Simplify and solve it for x:

2.4 + x = 0.25*(30+x),

2.4 + x = 7.5 + 0.25x,

x - 0.25x = 7.5 - 2.4,

0.75x = 5.1  ---->  x =  = 6.8.


Answer.  6.8 gallons of the pure disinfectant must be added.