###Human: Every year 20 ducks from a flock are killed but another 30 are born.  The original flock size is 100 ducks.  After 5 years they join with another flock of 150 ducks.  How many ducks are in the combined flock?, ### Assistant: The original flock gains 30-20=10 ducks per year
So after 5 years they have gained 10*5=50 ducks
So the original flock has 100+50=150 ducks
So the combined flock has 150+150=300 ducks 
 So the answer is :  300