COMEDK Exam 2019 Solved Paper

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 ‌‌=‌

‌ Total time ‌

‌ Number of half ‌−‌ life ‌

t1∕2‌=‌

60

3

=20‌min