Answer:
Step-by-step explanation:
mass of each part = 6 / 3 = 2 kg .
momentum of each of given part = 2 x 20 = 40 kg m/s 
Two momentum of 40 kg m/s , acting at angle 60 degree .
Resultant momentum = 2 x 40 cos 30 = 69.28 kg m/s 
The third mass will have equal and opposite momentum to this momentum , following law of conservation of momentum . 
If its velocity be v .
2 x v = 69.28 
v = 34.64 m /s 
Third mass will have velocity of 34.64 m /s 
Total kinetic energy of all three mass 
KE = 1/2 x 2 ( 20² + 20² + 34.64² ) 
= 400 + 400 + 1199.92 
= 1999.93 J .