For first order reaction t1/2=0.693/K
where, t1/2 is half life and K is rate constant
Given half life=100 s, therefore K=0.00693
Kt=2.303log10A0/A
Given A=10/100A0=0.1A0
putting these values in this equation
0.00693×t=2.303log1010
0.00693×t=2.303×1
t=2.303/0.00693
t=332.32 sec apox and