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more questions about exponential function for a decay problem

Andrew measure the amount of a very unstable substance to be 100 moles.  The half life of this substance is 3 days (after 3 days, half is gone).
 
This is what I have so far to solve this table.  ( y = amount of substance and x is time in days)
 
                                    Days                       Amount
 
                                  -3(3 days prior)           200
                                       0                           100
                                       3                            50
                                       6                            25
                                       9                            12.50
                                     12                              6.25
 
(This is using y = 100(.5) ^x/3
 
 
How would I graph this information from the table?
 
How do I calculate the expected amount of substance if Andrew had take his measurement 9 days earlier?  Show the calculations and consider the trend of the graph from above.
 
              Would the calculation be   y = 100(.5) ^x/9  ?
 
thanks
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1 Answer

NO!  The half life is 3 days at all times.  This implies that the material diminishes by 1/2 every 3 days.  Conversely, the material DOUBLES as you go back in time in 3 day increments.
 
Therefore, the amount of material:
 
6 days ago: 400 moles
9 days ago: 800 moles
 
The formula (exponent) wont change
 
Enter 100*.5^(x/3) in google search and it will plot it for you!  You can write a program to do this or just place dots on the graph (1/2-life numbers) and fill in the graph yourself.