Firemen use one hose to spray water on a house fire at a rate of $12$ gallons per minute. After $20$ minutes, the firemen connect a second hose to a hydrant to double the total spray rate. It takes $100$ minutes to put out the fire. The graph below shows the spray rate as a function of time.
**How much water do the firemen use to put out the fire?**
[[☃ input-number 1]] gallons