In this problem, we write the momentum conservation law in the form

     =     (1)


not because the collision is elastic.  We write it because the momentum conservation 
is the universal conservation law, which is held AT ANY COLLISION.


In this problem, the momentum conservation law is in this specific form (1), 
because the second ball was at rest before the collision,
while the first ball was motionless after the collision by the condition.


From (1), we find 

     = ,   =  = 16.304 m/s  (rounded).    ANSWER