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=<T>subtract(30, 20)=10 ducks per year
So after 5 years they have gained 10*5=<T>multiply(10, 5)=50 ducks
So the original flock has 100+50=<T>add(100, 50)=150 ducks
So the combined flock has 150+150=<T>add(150, 150)=300 ducks
