What would happen if a solution containing 45g of potassium chloride and 35g of solution of sodium chloride in 100g water were cooled to 0 degree c

How much of a substance we can dissolve in water depends
upon the temperature of the water.  It is measured as grams of substance per 100 grams
of water and is called soluability.  If there is less of
the substance it will all dissolved.  If there is more then either one of two things
will happen:  if it is cooled quickly then some of the substance will precipitate out;
if it is cooled slowly and not disturbed it will hold on to the extra amount in
solution.  This is called a supersaturated
solution.

So lets see what will
happen:

The soluability of NaCl is 35.7 grams at 0 C and
35.8 at 10 C.  Since you have only 35 grams of NaCl this is less than the limit so it
should stay in solution.

The soluability of KCl is 28 grams
at 0C and 31 grams at 10 C.  Since you have more KCl than the limit to begin with it
won't all be dissolved.  (You'd have to heat it to 60 C to dissolve all 45 grams!) As
the temperature cools the water tends to hold less and less KCl.  You will either have a
supersaturated solution or some will precipitate out until it holds only 28
grams.

I am assuming that you are not trying to mix the two
in 100 grams of water.  The fact that they are both chlorides will make the problem much
more complicated.