Question

Initally $480$ $\alpha$ -particles per minute is being emitted by a radioactive substance. This number reduces to $240$ after $2$ hours, The number of $\alpha$ -particles being emitted per minute after next $4$ hour will be

• A. $0$
• B. $60$
• C. $80$
• D. $120$

Answer 1

Answer: (B)

$60$

After next $4$ hour total time from initial position is $6$ hour.

So it is equivalent to $3$ half life.

Particle emitted per minute after $6$ hour is $=\frac{480}{2^3}=60$