Question 1175630
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<pre>

    {{{(0.18*20)/(20+w)}}} = 0.1


    3.6                    = 0.1*(20+w)

    3.6                    = 2 + 0.1w

    3.6 - 2                = 0.1w

    1.6                    = 0.1w

      w                    = {{{1.6/0.1}}} = 16.


<U>ANSWER</U>.  16 mL of water should be added.


<U>CHECK</U>.   {{{(0.18*20)/(20+16)}}} = {{{3.6/36}}} = 0.1 = 10% concentration.   ! Correct !
</pre>


It can be solved mentally, too.



<pre>
    20 mL of the 18% solution contain  0.18*20 = 3.6 mL pure alcohol.


    To have 10% solution, the total volume must be  36 mL;


    hence,  16 = 36-20  milliliters of water must be added.
</pre>


The second solutions provides THE SAME answer.