In English, when there are fractional expressions such as a half of, a part of, a percentage of, a majority of, a third of, etc., the verb agreement depends on the plurality of the nouns that follow them. For example:
50% (A half, A third, A part, etc.) of the university was destroyed by fire. (Not were because university is singular.)
50% (A half, A third, A part, etc.) of the parents were at the meeting. (Not was because parents are plural.)
[Examples are from Subject-Verb Agreement at elc.polyu.edu.hk]
If you change the word back to "back part (of his pants)" or "back half (of his pants)", the two phrases contain the words part and half and the rule applies. Therefore, the verb should agree to pants, not to back.
However, the word side seems trickier than back in "the back of his pants". A pair of glasses has only two sides and if it is the one side of the two sides which is bent, it should use a singular verb as in
The side of my glasses is bent.
Best Answer
...is correct.
...is always incorrect.
Since "task" and "job" are similar enough, a substitution helps to show the error. Incorrect: