Correct Answer - 1845 years
N
N0
=80%or0.80
t1/2=5730years
k=
0.693
t1/2
=
0.693
5730years
=1.21×10-4year-1
k=
2.303
t
log
N0
N
1.21×10-4=
2.303
t
log
1
0.8
T=
2.303
1.21XX104
log1.25
=
2.303
1.21×10-4
log1.25
=
2.303
1.21×10-4
×0.0969=1845years
Alternatively
Use direct relation,
t80%
t1/2
=
log(100)
80
0.3
t80%=
5730years×log0.8
0.3
≈1845years.