Section 1.3: Functions

Solution
No two different students of the class can have same roll number. Therefore, \(f\) must be one-one. We can assume without any loss of generality that roll numbers of students are from 1 to 50. This implies that 51 in \(\mathbb{N}\) is not roll number of any student of the class, so 51 cannot be image of any element of \(X\) under \(f\). Hence, \(f\) is not onto.