if n=m
then 1 cannot equal 2 because then n and m are not the same
The whole point is to figure out what is wrong with the string of equations as one leads into the other. It's a puzzle, you are meant to find the flaw and explain WHY 1=2 although the transition from one formulation of the equation to the other is mathematically correct.
Look at the equation and prove to me why it is wrong.