apply the law of conservation of linear momentum. momentum of first part = 1 x 12 = 12 kg ms-1Momentum of the second part  = 2 x 8 = 16 kg ms-1 Resultant monmentum= [(12)2 +(16)2]1/2 = 20 kg ms-1The third part should also have the same momentum.Let the mass of third part be M, then 4 x M = 20
M = 5 kg