IR-MO 2004 Problem 15

This problem is easy but nobody solved it. point $A$ moves in a line with speed $v$ and $B$ moves also with speed $v'$ that at every time the direction of move of $B$ goes from $A$ .We know $v \geq v'$ .If we know the point of beginning of path of $A$ , then $B$ must be where at first that $B$ can catch $A$ .