Each "x" represents one tree that produced that amount of sap in gallons.
To find the total amount of sap produced, add all of the amount of sap produced from each tree
3 trees produced 1/4 gallons of sap
2 trees produced 3/8 gallons of sap
4 trees produced 5/8 gallons of sap
1 tree produced 1 gallon of sap
You can do this:

Make all the denominators the same

5 gallons
Or you could have done:

Make all the denominators the same

5 gallons