Which statistical tests can I conduct to analyse the trend of series data?

I have a dataset that measures the a student time spent working on a mathematics question. My dataframe looks a little something like this:

I have times for 20 questions for about 200 participants. Now I have observed an overall decrease in time spent per question, as is shown in the figure below:

I would like to accompany graph this with a statistical measure of negative tendency.

I don't think I should use a correlation statistic as the question number is a categorical variable.

I maybe could do a OLS regression, with X being a the question number and y being the time spent per question, but I am not sure how to interpret the result.

What else could I try?