Question

A 40 kg shell is lying at speed of 72 km/h . it explodes into pieces, one piece of mass 15 kg stop .Calculate the velocity of the other piece.

Answer 1

Please see the attachment!
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Answer 2

by using the law of conservation of momentum we get 

m1 u1+ m2 u2 = m1 v1 + m2 v 2

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as the object was a single one before exploding we can take it as a single initial momentum

m1 u1+ m2 u2 = M x U

M x U = m1 v1 + m2 v 2

as one of the body stops its momentum will be zero 
therefore 
we can say that m1 v1 = 0

72 km/hr = 20 m/s

40 x 72 = m2 v 2

mass of second body = 40 - 15 = 25

40 x 20 = 25 v 2

800 = 25v2

v2 = 800/25

32  m/s