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? Think carefully first, then make a decision:
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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 answer is 300.
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