SOLUTION: A chemical solution contains 30% water and 70% alkali. What quantity of water needs to be added to 6 litres of solution so that water content become 40%?

Let the amount of water be x

                                  Liters
                                    of
                      Percent      Pure
 Type       Liters     alkali     alkali   
  of         of         as a        in
liquid      liquid    decimal     liquid 
-------------------------------------------
alkali        6        0.7      6(0.7)=4.2
water         x        0.0      x(0.0)= 0
-------------------------------------------
mixture      6+x       0.4      0.4(6+x)

 The equation comes from the last column.

                4.2 + 0 = 0.4(6+x)

Solve that and get 4.5 liters of water.

Edwin