Amount left = Initial Amount/2n Amount left/ Initial Amount = 2-n =>0.25 = 2-n =>1/4 = 2-n =>2-2 = 2-n => n=2 Hence the element will take 2 x 5730 =11460 years to reach 25% Solution Amount left = Initial Amount/2n Amount left/ Initial Amount = 2-n =>0.25 = 2-n =>1/4 = 2-n =>2-2 = 2-n => n=2 Hence the element will take 2 x 5730 =11460 years to reach 25%.