The property that is being described in the statement "The sum of the components of anything equals the whole thing" would be the Partition Postulate. It is simply the whole is equal to the sum of its parts. For instance we have a line where it contains points W, X, Y and Z, then WX + XY + YZ = WZ.
Answer:
Eric's expression is :

and Andrea's is :

In Eric's expression, 20 represents the initial amount of substance with which he has started the experiment.
is the amount of substance left after each time period (in this case, each week).The variable w in this case represents the number of weeks.
Andrea's expression can be written as :

The one outside of parentheses represents the initial amount of the substance. The one inside of parentheses represents 100% of the original amount of the substance. 0.5 represents the 50% of the substance that is lost each time period. The variable w in this case represents the number of weeks.
-2n-9-10n
69a-10
Hope this helps