Two particles are performing simple harmonic motion in a straight line about the same equilibrium point. The amplitude and time period for both particles are same and equal to $A$ and $T$, respectively. At time $\mathrm{t}=0$ one particle has displacement $A$ while the other one has displacement $-\frac{\mathrm{A}}{2}$ and they are moving towards each other. If they cross each other at time $t$, then $t$ is: