How much of pure antifreeze and how much of 40% antifreeze should be mixed 
              to make 4 liters of the 40% antifreeze.

Let x be the volume of the pure antifreeze.

Then the volume of the 40% antifreeze should be  (4-x) liters.


The basic equation is

    x + 0.4*(4-x) = 0.7*4

saying that the sum of antifreeze volumes in ingredients is equal to the volume of the antifreeze in the mixture.


From equation, express x and calculate


    x =  = 2 liters.


ANSWER.  2 liters of the 70% antifreeze should be mixed with 2 liters of the pure antifreeze.