SOLUTION: A nurse has 100 ml of a solution that is 10% salt. How much sterile water must be added to make an 8% salt solution? (BTW I want how much sterile water needed to add)

100 ml of the 10% salt solution contain  0.1*100 = 10 grams of dissolved salt.


In order 10 grams of dissolved salt make 8% solution, the volume of the solution must be   = 0.08, i.e.  volume =  = 125 mL.


So you need add 125-100 = 25 mL of the sterile water.    ANSWER

Let W be the volume of the sterile water to add.

Then the total volume is  (100+W) milliliters,  and your equation is


     = 0.08,


saying that the final concentration is 8%.


Solve it.  First step multiply both sides by (100+W).


    0.1*100 = 0.08(100+W)

    10      = 8 + 0.08W

    10 - 8  = 0.08W

     2      = 0.08*W

    W       =  = 25.


You get the same ANSWER : 25 mL of the sterile water should be added.