Two persons P and Q crosses the river starting from point A on one side to exactly opposite point B on the other bank of the river. The person P crosses the river in the shortest path. The person Q crosses the river in shortest time and walks back to point B. Velocity of river is 3kmph and speed of each boat is 5kmph w.r.t river. If the two persons reach the point B in the same time, then the speed of walk of Q is