2.0 g of a mixture of and was heated when its weight reduced to 1.876 g. determine the percentafe composition of the mixture

For example in this ap chem lab i have to calculate the percent of Na2CO3 and the percent of NaHCO3 in this mixture. I have 4g of the mixture. Percent comp is total mass of element/ molar mass of the compound but how can i calculate that with the compound i have here? Also with the data of 4g original mixture and 3.19g after heating, how can i calculate the atomic mass of Na2CO3

The answer to this question would be: cs 

Electron affinity is determined by the amount of energy released/spent for the atom to add an electron for neutral atoms in gas state. 
Lower periodic table means a bigger radius atom. In this case, the distance between the electron to the nucleus is further too, so the affinity is expected to become lower.

In this question, the K and Cs are located in the first column of the periodic table, but Cs located lower. So, the Cs that located lower in periodic table should have a lower affinity.