1. **Problem Statement:** We are given that 30 students like Maths, 25 like Science, and 10 like both Maths and Science. We need to find how many students like only Maths. 2. **Formula and Explanation:** To find the number of students who like only Maths, we use the principle of set theory. The number of students who like only Maths is given by: $$\text{Only Maths} = \text{Total Maths} - \text{Both Maths and Science}$$ This is because the students who like both subjects are counted in the total Maths count, so we subtract them to get only those who like Maths exclusively. 3. **Calculation:** $$\text{Only Maths} = 30 - 10 = 20$$ 4. **Answer:** The number of students who like only Maths is 20. Therefore, the correct choice is **a. 20**.