A train of length L move with a constant speed $V_t$. A person at the back of the train fires a bullet at time $t=0$ towards a target which is at a distance of D(at time $t=0$) from the front of the train(on the same direction of motion). Another person at the front of the train fires another bullet at time $t=T$ towards the same target. Both bullets reach the target at the same time. Assuming the speed of the bullets $V_b$ are same, the length of the train is?