Answer : The time required would be, 60.0 hours
Explanation :
Half-life = 15 hr
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 = initial amount of the reactant = 0.010 mol
a - x = amount left after decay process = 6.25 × 10⁻⁴ mol
Now put all the given values in above equation, we get


Therefore, the time required would be, 60.0 hours