Use the law of constant composition to complete the following table summarizing the amounts of nitrogen and oxygen produced upon the decomposition of the sample of dinitrogen monoxide.

:  N2O has a molar mass of (2)14 + (1)16 = 40g/mol N2O (both are slight approximations) 
this means any given sample of N2O is (16/40) = 40% Oxygen and 60% Nitrogen 

sample B is 4.35g 
4.35g * 0.40 = 1.74 g O 
4.35g * 0.60 = 2.61 g N 

samples C and D require a tiny bit of algebra: 
Sample C 
(unknown sample size C)(0.40) = 1.05g O 
therefore (1.05g)/(0.40) = x =2.625g N2O 
(2.625g)*(0.60)=1.575g N 

Sample D 
(unknown sample size D)(0.60) = 1.31g N 
therefore (1.31g)/(0.60) = 2.183g N2O 
(2.183g)*(0.40)=0.873g O 

Once youve calculated the % mass of each element in the compound, you can use them as ratios. If something is 25% element X, and a random sample has 0.5g X, then you know that the sample must be 2 grams in total because 25% of it weighs 0.5g, which is 25% of 2 grams. simple algebra. 

sample 1 = M 
(1/4)M = 0.5g, therefore M =?