Question 257409
Let x be the number of students in the class
Let (x-1) be the number of cards each student sends (everyone but themselves)
x(x-1)=1,560
x^2-x-1,560=0
(x-40)(x+39)=0
x-40=0
x=40 students are in the class.
Proof:
40*39=1,560
1,560=1,560