Answer:
% composition of Hg = 93 %
% composition of O = 7 %
Step-by-step explanation:
Data Given:
mass of total sample = 14.2 g
mass of mercury (Hg) = 13.2 g
percent composition of Hg =
percent composition of O =
Solution:
First find mass of oxygen
As this compound only have two components that is oxygen and mercury
So 
upon decomposition it gives 13.2 g mercury 
now to find mass of oxygen we have to subtract mass of mercury from total mass of sample
  Mass of oxygen = Total mass of sample - mass of Mercury
 Mass of oxygen = 14.2 g - 13.2 g
 Mass of oxygen = 1 g
Now to find percent composition of Hg and Oxygen
% composition = component mass / total mass of sample x 100. . . .(1)
For % composition of mercury Hg 
Put values in equation 1
 % composition of Hg = 13.2 g / 14.2 g x 100
 % composition of Hg = 93%
For % composition Oxygen (O)
Put values in equation 1
 % composition of O = 1 g / 14.2 g x 100
 % composition of O = 7 %