SOLUTION: If you have 1.75L of an 18% alcohol solution, how much pure water must you add in order to decrease the concentration to 5%?

The amount of the pure alcohol in 1.75 L of the 18% solution is  0.18*1.75 liters = 0.315 liters.


This volume of the pure alcohol will not change and will remain the same after adding water.


So, you need to add, let say, x liter of water to get


    = 0.05   (which is 5% of concentration of the final mixture)


So, you need solve this equation.


For it, multiply the both sides of the equation by (0.315+x) to get


    0.315 = 0.05*(0.315 + x).


Simplify and solve for x:


    0.315 = 0.05*0.315 + 0.05x

    0.315 - 0.05*0.315 = 0.05x


    x =  = 5.985.


Answer.  You need to add  5.985 liters of water.


Check.    = 0.05.    ! Correct !