The first time, you rode an average of 12.5 mph. The second, you rode an average of 16. In the first instance, you rode 1.6 hours (36 minutes = .6 of an hour), and in the second run, you rode 1.25 hours. Divide both by 20 miles and you'll get these results.

And avoid appearing on NPR's Whatdya Know, with Michael Feldman, because you will get the question wrong if the category is Things You Should Have Learned in School. :-)