The half-life of a substance in a certain enzyme-catalysed reaction is 138s. The time required for the concentration of the substance to fall from 1.28 mg/L to 0.04 mg/L is:

Enzyme catalysed reactions are initially follow first order kinetics when concentration decreases from 1.28 mg/L to 0.04 mg/L.
1.28mg/L / 0.04mg/L = 32 = 2⁵
Then five half-lives completed.
Number of half-lives = 5
So, time required = 5 × 138 = 690s
The time required for the concentration of the substance to fall from 1.28mg/L to 0.04mg/L, is 690s.