A radioactive substance reduces to 25%of its original mass in 1000years. Find the half life period of the substance.
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A=A0(1/2)(T/Th)     A0=initial amount      A=amount left       T=time elapsed       Th= half life period    
Let A0=m   then  A=m/4     So,   m/4=(m)[(1/2)(1000/Th) ]       1/4=[(1/2)(1000/Th) ]            (1/2)2=[(1/2)(1000/Th) ]
So,2=1000/Th   So,Th=500years
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