21. Radioactive decay is modeled with the exponential function , where t is time, is the amount of material at , is the amount of material at time t, and k is a constant. Gallium-67, which has a half-life of 3.261 days, is used for tracing cancer. If 100 milligrams are present at , determine the amount (rounded to the nearest tenth of a milligram) that is left after 7 days. Solve the problem by writing a program in a script file. The program first determines the constant k, then calculates , and finally rounds the answer to the nearest tenth of a milligram.