There are three professional swimmers A, B and C who can swim at speed of 11 km/hr, 8 km/hr and v km/hr respectively. A and B move from P to Q whereas C from Q to P. Now, B starts from P and swims towards Q 50 km away. Speed of the current (from P to Q) is 2 km/hr. A and C start half an hour later than B. What should be the speed of C so that all three meet together?