The building of Facultad C. C. Matemáticas is really close to the accommodation. You only need to walk down the street Leonar Prieto Castro, and you will arrive to Plaza de Ciencias, where the building is located.

How to get Facultad de C. C. Matemáticas from Colegio Mayor:

Universidad Complutense de Madrid
Facultad de Ciencias Matemáticas
Plaza de las Ciencias 3
28040 Madrid
Room Miguel de Guzmán