Two swimmers $A$ and $B$ start swimming from exactly opposite points on the separate banks of a river of width $8\ km$, flowing with a speed of $3\ km/h$. If speed of both the swimmers w.r.t to ground is $5\ km/hr$ then what is the distance travelled by $'A'$ till they meet? (Assume that their velocities w.r.t. to river are perpendicular to river flow).