CGPET 2015 Solved Paper

Number of half - lives in 40 min
n=

40

T 1 2

=

40

10

=4
Amount of the substance remaining after n half - lives
N′=N0(

1

2

)n=N0(

1

2

)4=

N0

16

where, N0 is original amount of the substance.
Amount of the substance decayed in 40 min.
N′=N0−N=N0−

N0

16

=

15

16

N0
=(

15

16

×100%)‌of‌N0
=15×6.25%ofN0
=93.75%ofN0