Question

A mixture consists of two radioactive materials $A_1$ and $A_2$ with half lives of $20\,s$ and $10\, s$ respectively. Initially the mixture has $40 \,g$ of $A_1$ and $160 \,g$ of $A_2$ The amount of the two in the mixture will become equal after

• A. 60 s
• B. 80 s
• C. 20 s
• D. 40 s

Answer 1

Answer: (D)

40 s

Answer 2

For $A,\, 40\, g \stackrel{20 s }{\longrightarrow} 20 g \stackrel{20 s }{\longrightarrow} 10 g$
For $B,\, 160\, g \stackrel{10 s }{\longrightarrow} 80 g \stackrel{10 s }{\longrightarrow} 40 g \stackrel{10 s }{\longrightarrow} 20 g \stackrel{10 s }{\longrightarrow} 10 g$