The first question is.
6 + 6 = 12
n is the same thing on both sides of the equation
6 + 6 - n = 12 - n
You are taking an equal amount off both sides so the left and right sides are both equal still. Try it. Let n = 0.5
6 + 6 - 0.5 = 12 - 0.5
6 + 5.5 = 11.5
11.5 =? 11.5
They are both equal. In algebra a letter in an equation like this one can't change values if that letter is on opposite sides of the equation.
Six
9.5 y + 600 < 900
9.5 y = 300
y < 300 / 9.5
y < 31.57
If there's no sales tax where he lives, he can take 31 students. If it is possible to take 0.57 of a student then he can take 32 (but of course it isn't possible).