Question 385423
N= 1/2 (x squared minus x) 
N= 78 
------
Solve for "x":
(1/2)(x^2-x) = 78
x^2-x = 156
---
x^2-x-156 = 0
Factor:
(x+12)(x-13) = 0
Positive solution:
x = 13
=======
Cheers,
Stan H.