Two persons P and Q crosses the river starting from point A 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 the shortest time and walks back to point B. Velocity of river is $3kmph$ and speed of each boat is $5kmph$ w.r.t. water. If the two persons reach the point B in the same time, then the speed of walk of Q is