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? Give me reasons, before answering the question

Show your work and conclude with the exact phrasing "Therefore, the final answer is [answer]." where [answer] is just the final number or expression that solves the problem.
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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 final answer is 300.