There is actually no error at a + b - c = 0. By its own, its
just a simple linear eqn with 3 variables (a,b and c) amounting to
0. For e.g. a = 1, b = 1, c = 2, therefore a + b - c = 0. The
error/humor lies in the way that the joke attempts to solve the eqn
by dividing away the variables and associating the final answer
with a ludicrous assumption: 3=4.
Simpler form of the joke, Three is equal to four:
I.e. 3x = 4x. "Remove the same term left and right", which in this
case is "X", divide both sides by "X" and therefore 3 = 4?
