Example 2: The data to the right show radioactivity measurements taken at onehour intervals. Even though the data graph (the solid dots in graph below) has a shape similar to that of a decaying exponential, a single exponentialdecay model (the circles in the graph below) does not fit the data well. Test whether the data could be fit well by the sum of two exponential models. If so, report the two decay rates and the relative activity of the two components.
Solution:
To fit the data to a model based on the sum of two basic exponentialdecay models, four parameters will be needed (the initial value and growth/decay rate for each of the basic models), so the formula placed in C3 will be “=$G$3*(1+$G$4)^A3+$G$5*(1+$G$6)^A3”, which will be spread down column C beside all the data values. Set the initialvalue parameters G3 and G5 to about 600 (half the first data value). Set the growthrate parameters G4 and G6 to different negative values (such as –10% and –30%) that make the model roughly match the data. Then use Solver to minimize the sum of squared deviations in H12 by changing the G3:G6 range of parameters. This produces the results below: 
Hours 
Activity 
0 
1333 
1 
799 
2 
513 
3 
359 
4 
270 
5 
220 
6 
191 
7 
173 
8 
154 
9 
145 
10 
137 
11 
134 
12 
122 
13 
113 
14 
109 
15 
104 
16 
95 
17 
94 
18 
88 
