The activity per ml of a solution of radioactive substance is x. How much water be added to 200 ml of this solution so that the acitivity falls to x/20 per ml after 4 half-lives ?
(A) 100ml
(B) 150ml
(C) 80ml
(D) 50ml.
Correct Answer is: (D) 50ml.
V = 250ml.
So added volume of water = 50ml.