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? Let's answer step by step:
In 7 weeks 5 * 7 = 35 goldfish die. In 7 weeks, Martin purchases 3 * 7 = 21 goldfish. In 7 weeks, Martin will have 18 - 35 + 21 = 4 goldfish. The answer: 4.