Assertion : An object has been given two velocities $\overrightarrow{v_1}$ and $\overrightarrow{v_2}$, then it has a resultant velocity $\overrightarrow{v }=\overrightarrow{v_1}+\overrightarrow{v_2}$.

Reason : $\overrightarrow{v_1}$ and $\overrightarrow{v_2}$ should be velocities with reference to some common reference frame.

The resultant of the two velocities $\overrightarrow{v_1}$ and $\overrightarrow{v_2}$ is the vector addition using parallelogram law of vector addition. $\overrightarrow{v }=\overrightarrow{v_1}+\overrightarrow{v_2}$ 

Both of the velocities must be from same frame of reference, for example: if a object is given two velocities and if one of the velocity is measured from a moving frame of reference and other from a stationary frame of reference. the addition of these velocities will not give the correct value of velocity of object.