Answer
4kg
Work Step by Step
Looking at Disk C, we have to find all the forces acting on it. We have T2 acting upwards with a magnitude of 49N. We also have T3 acting downwards as is Mc*g. Since the disk is not moving, it has no acceleration, and therefore the net Force (F=ma) is also 0. This means the sum of all the forces must be 0.
0 = T2 - T3 - Mc*g. Since g = 9.8, we find that Mc = 4kg
