Odd that when you simplify Harry’s argument in order to make it more understandable, with the actual numbers in the two arguments being identical when you work them out, Harry can no longer recognize it as his own argument.
Vaughan, as an educator you are certainly aware of a corollary:
It is always harder to evaluate a wildly incorrect solution to a homework problem than it is to grade something that another student has some sort of grasp on. In the former case, you often have to be a mind-reader.