CuCl2 is Dichloride Copper, or Copper II Chloride.
In this Molecule, there is 1 atom of Copper for every 2 atoms of Chlorine.
If you go to the periodic table, you'll see that copper has a mass of 63.5 amu, and the chlorine 35.45 amu.
Which mean that the mass of CuCl2 is 63.5 + 35.45*2 = 63.5 + 70.9 = 134.4 g/mol.
Then, you find the ration of the mass of each atom and multiply by 100.
Ratio of Copper:
and you get about 47.2%
Ration of chlorice (Cl2):
and you get about 52.8%.
So, the percent composition of CuCl2 is 47.2% of Copper and 52.8% of chlorine.
Hope this Helps :)