To determine the amount of water in the first 8 beakers, you will subtract the 19 ml that are in the 9th beaker from the total to see how much water was actually put into the first 8 beakers.
91 ml - 19 ml = 72 ml.
The 72 ml are divided evenly between 8 beakers.
72/8 = 9 ml
There would be 9 ml of water in each of the first 8 beakers.
