Pidgeon hole principle problem

A large calculus class has n students, with n between

100 and 200. Each student is identified with one of the numbers 1, 2,

. . . , n. The teacher randomly chooses three students to report on their

performances. When he entered the first student number on the computer,

his assistant on seeing this on the screen, immediately says that

the probability that the other two numbers are both smaller than this first

number is exactly 50-50.

What is this first number, and how many students are there in the

class? Thanks for your help.