Identify the limited topic of the following topic sentence.
word problems are the most difficult assignments in math classes.

Based on the given sentence above, the limited topic is "word problems". This topic limits or specifies the broader topic which is "assignments in math". Limited topic establishes the limit of what the writer is going to write about in reference to a broader topic. Hope this answers your question.

Not sure put the answer in the comments so this person doesn't get there question answered