80. In a junior high school, 100 students and teachers from two graduating classes are taking a graduation photo on the steps. The photographer wants to arrange them in a trapezoidal formation with fewer people in the front rows and more in the back rows (number of rows $\geqslant 3$), and requires that the number of people in each row must be consecutive natural numbers, so that each person in the back row can stand in the gap between two people in the front row. How many arrangements satisfy the above requirements? $\qquad$
answer: 2