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Answer:
6400 years
Explanation:
Given that;
N/No =(1/2)^t/t1/2
No = amount of radioactive material originally present
N = amount of radioactive material present at time t
t= time taken for N amount of radioactive material to remain
t1/2 = have life of the radioactive material
2.5/40 = (1/2)^t/1600
1/16 = (1/2)^t/1600
(1/2)^4 = (1/2)^t/1600
4 = t/1600
t = 4 * 1600
t = 6400 years