I know the final answer to the physics question below about resultant speed, but

please tell me the solution? A 20 g bullet moving at 100 m/s is fired at a 1 kg block of wood initially at rest and free to move.The bullet emerges from the other side at a speed of 100 m/s.What is the resultant speed of the wood block?

** I know the final answer is 18, but I don't know the solution
Please help me and answer me seriously and do not tease me please
 
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