The half-life of a radioactive element is 100 minutes. The time interval between the stages to 50% and 87.5% decay will be:

ΔT=t1−3t1/2=(100−300×100)=200minAlternatively:t87.5%t50%=log⁡100(10a0−87.5)log⁡100100−50t87.5%=t50%×log⁡10012.5log⁡2=100×(log⁡100−log⁡125+log⁡10)0.3=100×(2−3log⁡5+1)0.3=100×(3−3×0.7)/0.3=100×0.90.3=300min t87.5%=t1/2≡300−100=200min