Question

What is the percent composition of CuCl2?

Answer 1

$CuCl_2$
There is 2 times more $Cl$ than $Cu$.

So for 1 :
$Cu$ : $\frac{1}{3}$
$Cl$ : $\frac{2}{3}$

So percent composition :
$Cl$ ⇒ $66$%
$Cu$ ⇒ $33$%

Answer 2

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: $\frac{63.5}{134.4} * 100$ and you get about 47.2% 
Ration of chlorice (Cl2): $\frac{70.9}{134.4} * 100$ 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 :)