The fourth-grade class at Valley Elementary School has $35$ students: $20$ taking maths class, $11$ taking both the writing class and the maths class, and $10$ taking neither of the classes. How many students are taking the writing class only? 
Overseas Competition->Knowledge Point->Counting Modules->Permutations and Combinations->Combinations Overseas Competition->Knowledge Point->Counting Modules->Inclusion-Exclusion Principle->Inclusion-Exclusion Principle for Two Sets