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=
<gadget id="calculator">30-20</gadget>
<output>10</output>
10 ducks per year.
So after 5 years they have gained 10*5=
<gadget id="calculator">10*5</gadget>
<output>50</output>
50 ducks.
So the original flock has 100+50=
<gadget id="calculator">100+50</gadget>
<output>150</output>
150 ducks.
So the combined flock has 150+150=
<gadget id="calculator">150+150</gadget>
<output>300</output>
300 ducks.

<result>300</result>