Let N be the amount of substance left, N0 be the total amount of substance. The amount of substance left after N half-life is given by ‌
N
N0
=‌
1
2n
Hence, ‌‌‌
30
240
=‌
1
2n
(Since, decay rate proportional to amount of substance) ⇒‌‌2n=8⇒n=3 Number of half-life passed =3 Time taken for the substance to decay from 240 count rate to 30 count rate =1h=60‌min ‌ Half-life ‌‌=‌