Online Learning Resources for Discrete Mathematics
There is a whole course being offered by Udacity*, Logic and Discrete Mathematics, which has interactive quizzes and homework assignments.

The course description is as follows:


  This course presents key concepts in discrete mathematics, specifically, elementary propositional logic and elements of enumerative combinatorics, elementary number theory, and graph theory, with partially ordered sets and lattices as a unifying concept.


*I am not affiliated with them in any way, but I have taken some of their prior courses