Let the tension in the rope be `T` and accleration of masses be `a`. The frictional force acting on 50kg block is 250N. And that on 20 kg block is 100N. Therefore we can say that, 300 - (250 + T) = 50a, and150 + T - 100 = 20a from here we get a = 100/70 or 1.4285 = 2.5 k , so k = 4/7