Question

The count rate from 100 cm³ of a radioactive liquid is c. Some of this liquid is now discarded. The count rate of the remaining liquid is found to be c/10 after three half-lives. The volume of the remaining liquid, in cm³, is 

(a) 20 

(b) 40 

(c) 60 

(d) 80

Answer 1

Correct Answer is: (d) 80

Initial count rate (CR) for 1 cm³ of liquid = c/100.

After 3 half-lives, CR for 1 cm³ of liquid = 1/8 x c/100 .

Let the volume of the remaining liquid = V cm³.

∴ CR of this liquid = V x c/800 = c/10

or V = 80.