The half-life of gold-198 is 2.7 days. After
amount of gold-198 will remain.
days, 1/4 of the original

8.1 days

Explanation:

Expression for rate law for first order kinetics is given by:

where,

k = rate constant

t = age of sample

a = let initial amount of the reactant = x

a - x = amount left after decay process=  

a) to find rate constant

Half life is the amount of time taken by a radioactive material to decay to half of its original value.

b) for completion of one fourth of reaction

Thus after 8.1 days , one fourth of original amount will remain.

d.) circle motion requires a force to give it a centripetal acceleration.