"Were" should be "was" as it is referring to the word "average," a singular quantity. "Were" would be used if the author was referring to multiple averages.

No values regarding sugar percent concentration are specified with the three repeated experiments. It is clear that the author repeated the experiment with different concentrations of sugar water when the reader refers to the graph. These values need to be included in the description.

It is unclear if the author is trying to make a general statement about calculations or if the author is trying to relay what actually happened during the experiment. The verb "is" must be changed to "was" to be in the past passive tense. And, the "liquid" needs to be specified. For ease of understanding, the previous sentence can be combined with this one: "Then, the mass of the liquid within the beaker was calculated by subtracting the initial mass from the final mass."

The comma is unnecessary. Also, using the preposition "by" when referring to subtraction creates confusion. The described values should be flipped: "The volume was calculated by subtracting the initial volume of the water in the graduated cylinder from the volume of the graduated cylinder after the metal rod had been added. To further clarify, numeric values should be used.