Answer:
= 10 g
Explanation:
Half life is the time taken by a radioactive substance to decay by half of the original mass or initial amount.
Therefore, to get the new mass after decay;
New mass = Original mass × (1/2)^n ; where n is the number of half lives.
n = 60 seconds/20 seconds
= 3
Therefore;
New mass = 80 g × (1/2)^3
= 80 g × 1/8
= 10 g