Question: The local firefighters are doing a “fill the boot” fundraiser. Their goal is to raise $6300. After the first 3 hours, they have raised $2100.  For how many hours do they have to fundraise in total to reach their goal, assuming an equal amount raised in every hour?
Give me a clear explanation. 
Answer: 
The fireman raise 2100 / 3 = $700 per hour.
They have to fundraise for 6300 / 700 = 9 hours.