Question 922696
 INitially we have 60 millilitres of solution with 20 % salt
 so quantity of salt  in the solution  =  20% of 60 milliliters
                                       ={{{ (20/100)*60}}}
                                        ={{{(20/5)*3}}}
                                        ={{{4*3}}}
                                         = {{{12 }}} millilitres
let assume v milliliters of pure water is added to the salt solution
   so total quantity of salt solution now becomes   60 +v millilitres
               quantity of salt after adding water is 12 % of  (60+v) millilitres
but quantity of salt in the solution is remains same even after adding water to the salt solution
   so    12  =  12 % of (60+v)
          {{{ 12 = (12/100)*(60+v) }}}
             multiply with {{{(100/12)}}} on both sides
        {{{ 12*(100/12) = (12/100)*(60+v)*(100/12)}}}
                 {{{ 100=60+v}}}
                       subtract 60 on both sides
                      {{{100-60 =60+v-60}}}
                           40 =v
                          so v= 40 millilitres
result : quantity of water added  = 40 millilitres