two spherical balls a and b of masses 2kg and 4kg are moving along same line with velocities 2m/s and 4 m/s respectively .They collide and after collision the second ball moves with a velocity of 1.5 m/s find the final velocity of the first ball

Dear Student ,
Here in this question the given data is , 
Mass of the first body is 2 kg and second body is 4 Kg.Velocity of the first and second body before collision is 2 m/s and 4 m/s and the velocity of the second body after collision is 1.5 m/s.
Let us take that the velocity of the first body after collision is v' .
Therefore , according to the conservation of Linear Momentum ,
Momentum before collision = Momentum after collision
$$\begin{gathered} \left(2\times 2\right)+\left(4\times 4\right)=\left(2\times v\text{'}\right)+\left(4\times 1·5\right) \\ \Rightarrow 20-6=2v\text{'} \\ \Rightarrow v\text{'}=\frac{14}{2}=7 m/s \end{gathered}$$
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