In an out of control ceramics workshop, two clay balls collide in mid air and stick together. the first has mass 3.1 kg and collides with a second that is initially at rest. the composite system moves with a speed equal to one-third the original speed of the 3.1 kg ball. what is the mass of the second sphere?

We know that the first ball has a mass of 3.1 kg and moves at speed called V1 The second ball has a mass of M2 unknown and the both together moves a speed equal to V = V1/3

   The momentum keeps constant before and after balls collide, so:
 M1*V1 + M2*V2 = M*V
 3.1*V1 + M2*0 = (3.1+M2)*V1/3
 3.1*V1 = 3.1/3*V1+M2*V1/3
 3.1*V1-3.1/3*V1 = M2*V1/3
 (3.1-3.1/3)*V1 = M2*V1/3
 2*3.1/3 = M2/3
 M2 = 6.2
   Then the mass of the second ball is 6.2 kg

Is writing an option