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?
The original flock gains 30-20=<<30-20=10>>10 ducks per year
So after 5 years they have gained 10*5=<<10*5=50>>50 ducks
So the original flock has 100+50=<<100+50=150>>150 ducks
So the combined flock has 150+150=<<150+150=300>>300 ducks
#### 300