Respuesta :

mathmate
Looks like a tough one!

by fitting1/8*x^3-1/4*x^2+23/8*x+1/4, 
we have for
x N
1 3
2 6
3 10
5 24
and when x=4, N=63/4=15.75 (question does not require N to be an integer).

If I find a solution of integers, I will share with you.