Miscellaneous problems in mathematics

A man can ride a boat in calm water with u ms-1 continuous velocity. A dog can swim in calm water v ms-1 continuous velocity. The man and dog both stay on the same bank of a straight river on points A, B respectively which is flowing with V ms-1 continuous velocity of width a meters. The length of AB is d meters. AB is in the direction of V(Vuv).The man ride the boat across the river such that he can reach the point C which is situated directly opposite to the point A. Show that, the time taken by him to move from A to C is given by a√u2-V2 seconds.