To determine the number of gallons after 35 hours, we will read off the graph.
The y axis is given in gallons and the x axis is given in hours.
When looking at the graph, we can see that there is a straight line between 25 hours and 50 hours. This means that between these hours, the amount of gallons remained the same.
Since 35 hours is between 25 and 50 hours we can conclude that there were 3000 gallons in the pool after 35 hours.