A flask contains a mixture of compounds A and B. Both compounds decompose by fist order kinetics. The half-lives for A and B are $300 \mathrm{s}$ and $180 \mathrm{s}$, respectively. If the concentrations of A and B are equal initially, the time required for the concentration of A to be four times that of B (in S) is. (Use In $2=0.693$ )