s + n = 405
4s + 6n = 2002
Since we are looking for n, we are going to solve the first equation for the other variable, s.
s = 405 - n
Now, since s and 405 - n are equal, they are interchangeable.
4(405 - n) + 6n = 2002
1620 - 4n + 6n = 2002
2n = 382
n = 191 non-students