Don't we subtract instead of adding in titration

That's a great question, Elward.

In a titration, you typically subtract the initial burette reading from the final burette reading to find the volume of titrant used. This volume is what you then use in your calculations.

However, when you are asked for the average of multiple volumes of titrant used, you would add those individual volumes together and divide by the number of volumes, just like we did.

So, for the specific question of finding the average of the two readings you provided (13.23 and 14.36), the calculation remains the same: add them and divide by two.