Question

The half life of a radioactive substance is 20s, the time taken for the sample to decay by `7//8^(th)` of its initial value is
A. 20 s
B. 40 s
C. 60 s
D. 80 s

Answer 1

Correct Answer - c
After three half lives , the fraction of undecayed nuclei
`=(1/2)^3=1/8`
`therefore ` Time taken for the sample to decay by `(1-1//8)^(th)` or `7^(th)/8` of initial value.
`=3T_(1//2)=3xx20`=60 s