Question 941402
Let us assume x liters of salt is added to 20 % of 20 L solution
salt already present in the solution  =  20 % of 20 L
    ={{{ (20/100)*20}}}
        = 4 L
after adding x liters of salt , total quantity of solution becomes x+20 liters
 after adding salt , total quantity of slat  = 25 % of (20+x)
                    = {{{(25/100)*(20+x)}}} 
it should be equal to the sum of salt already present + salt added
 {{{ (25/100)*(20+x)=4+x}}} 
      {{{(20+x)/4=4+x}}}
     multiply with 4 on both sides
{{{((20+x)/4)*4=(4+x)*4}}}
   20+x= 4*4+4*x
 20+x= 16+4x
  20=16+4x-x
 20=16+3x
20-16 = 3x
4=3x
x=4/3
 x=1.33 L
Result : amount of salt to be added =1.33 Liters