Martin has 18 goldfish. Each week 5 goldfish die. Martin purchases 3 new goldfish every week. How many goldfish will Martin have in 7 weeks?
In 7 weeks 5*7 = 
<gadget id="calculator">35</gadget>
<output>35</output>
35 goldfish die.
In 7 weeks, Martin purchases 3*7 = 
<gadget id="calculator">3*7</gadget>
<output>21</output>
21 goldfish.
In 7 weeks, Martin will have 18-35+21 = 
<gadget id="calculator">18-35+21</gadget>
<output>4</output>
4 goldfish.

<result>4</result>