The half life of a substance in a certain enzyme catalyzed reaction is 138s. The time required for the concentration of the substance to fall from `1.28 mg L^(-1) to 0.04 mg L^(-1)` :

Half life of a certain zero order reaction, A rightarrow P is 2 hour when the initial concentration of the reactant, 'A' is 4 mol L^(-1) . The time required for its concentration to change from 0.40 to 0.20 mol L^(-1) is