In[1]:= eqtn = r'@tD == -0.1 * r@tD;

gensol = DSolve@eqtn, r@tD, tD;

parsol = DSolve@8eqtn, r@0D � 200<, r@tD, tD;

greqtn = r@tD �. parsol;

Plot@Evaluate@greqtnD, 8t, 0, 20<, PlotLegends ® "Expressions",

PlotRange ® 80, 200<, PlotLabel ® "Decay Model", GridLines ® 883<, 8<<,

AxesLabel ® 8"Time", "Radioactive material in Grams"<Dgrval = greqtn �. t ® 3

Out[5]=

0 5 10 15 20

Time

50

100

150

200

Radioactive material in Grams

Decay Model

200. ã-0.1 t

Out[6]= 8148.164<