Question

90 % of the active nuclei present in a radioactive sample are found to remain undecayed after 1 day. The percentage of undecayed nuclei left after two days will be 

(a) 85 % 

(b) 81 % 

(c) 80 % 

(d) 79 %

Answer 1

Correct Answer is: (b) 81 %

Equal fractions decay in equal times.

∴ if a fraction 0.9 remains undecayed after 1 day, a fraction (0.9)² = 0.81 will remain undecayed after 2 days.