Question

If rank of a student from last is 17th and 15th from beginning, then how many students are there?

Answer 1

If there are  N students,  the rank of the 1st student is 1 from beginning and N from the last.  This is an example to find the following rule:

The number of students = rank of a student from beginning + rank from last  - 1
     Here    number of students = 17 + 15 - 1 = 31