CGPET 2021 Solved Paper

Given, half - life period of a radioactive substance,
T

1

2

=5‌min
Total time, T=20‌min
Number of half - life,
n=

Total‌time(T)

Half−life(T 1 2 )

=

20

5

=4
After n half - life, the amount of substance undecayed,
N=N0(

1

2

)n
Fraction undecayed =

N

N0

=(

1

2

)4
∴ Fraction decayed =1−

N

N0

=1−(

1

2

)4=

15

16

Therefore, percentage decayed amount
=(

15

16

×100)%=93.75%