In the figure, pulleys are smooth and strings are massless, m1 = 1 kg and To keep m3 at rest, mass m3 should be
m3 is at rest. Therefore,
2T = m3 g …(i)
Further, if m3 is at rest, then pulley P is also at rest.
Writing equations of motion, m1g T = m1a …(ii)
T m2g = m2a …(iii)
Solving Eqs. (i), (ii) and (iii), we get
m3 = 1 kg