In a first order reaction, the concentration of the reactant, decreases from 0.8 M to 0.4 M in 15 minutes. The time taken for the concentration to change from 0.1 M to 0.025 M is

The concentration of the reactants decrease from 0.8 to 0.4 in 15 min i.e., t1/2=15 min, concentration from 0.1 M to 0.025 M will fall in 2 half lives so total time taken=2×T1/2=2×15=30 min.