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 $t = 0$ one particle has displacement $A$ while the other one has displacement $\frac{-A}{2}$ and they are moving towards each other. If they
cross each other at time $t$, then $t$ is :