Answer :
(1) The time passed by the sample is

(2) The amount left after decay process is 1.14 mmol.
Explanation :
Part 1 :
Half-life = 5730 years
First we have to calculate the rate constant, we use the formula :



Now we have to calculate the time passed.
Expression for rate law for first order kinetics is given by:

where,
k = rate constant =

t = time passed by the sample = ?
a = let initial amount of the reactant = 100 g
a - x = amount left after decay process = 100 - 25 = 75 g
Now put all the given values in above equation, we get


Therefore, the time passed by the sample is

Part 2 :
Now we have to calculate the amount left.
Expression for rate law for first order kinetics is given by:

where,
k = rate constant =

t = time passed by the sample = 2255 years
a = let initial amount of the reactant = 1.5 mmol
a - x = amount left after decay process = ?
Now put all the given values in above equation, we get


Therefore, the amount left after decay process is 1.14 mmol.