Let the rate =k[A]
At t=10 seconds
0.04=k[A1]
At t=20 seconds
0.03=k[A2]
∴, in 10 seconds concentration of A decreases by 0.01k[M]
Using first order kinetics equation,
10×k=ln(0.04/k0.03/k)
10×k=0.028
k=0.028
Half life of first order reaction =ln 2k
t1/2=ln 20.028=24.1 seconds