Question: Jay is making snowballs to prepare for a snowball fight with his sister. He can build 20 snowballs in an hour, but 2 melt every 15 minutes. How long will it take before he has 60 snowballs?
Find the solution. 
Answer: 
Every hour, Jay makes 20 snowballs.
At the same time, each hour 2 * 60/15 = 8 snowballs melt.
This means that each hour, Jay ends up with 20 – 8 = 12 more snowballs.
It will take Jay 60 / 12 = 5 hours to create enough snowballs.