Q. A mixture consists of two radioactive materials A₁ and A₂ with half lives of 20 s and 10 s respectively. Initially the mixture has 40 g of A₁ and 160 g of A₂. The amount of the two in the mixture will become equal after
A) 40 s
B) 60 s
C) 80 s
D) 20 s
Explanation
[Solution from source] After t seconds, A₁ = 40(½)^(t/20) and A₂ = 160(½)^(t/10). Equating: 4 = 2^(t/10 − t/20) = 2^(t/20), so t/20 = 2 and t = 40 s. Check: both become 10 g at 40 s. Answer (1).
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.