Walk to the street crossing just upstreet of the hotel and take the subway at "Argüelles", grey line, to "Ciudad Universitaria", then follow the blue line map OR walk further along to "Calle de la Princesa" (to your left coming from the hotel) to Moncloa square and catch bus 82 (to the mathematics stop). On september 1st, 2nd and 3rd you can also catch the university line, "G" that is somewhat faster.
