Each "x" on the number line seems to represent a tree that sap has been collected from. That's 15 trees. Furthermore, you know that the total amount of sap is 8 gallons, because it's stated in the question.
To get the amount of sap that would be collected from each tree if it had been distributed equally (also known as the average amount of sap collected from each tree), just do exactly that.
Mathematically, this means you would divide the 8 gallons by 15 trees, or 8 ÷ 15. The total amount of sap collected is equivalent to an average of eight fifteenths of a gallon per tree.
Hope that helps c: