If $4=5$, then $6=8\,$ (yes or no?) I had an argument with a friend about the statement in the title. I asserted that if $4=5$, then $6=8$, as you can derive any conclusion from a false statement. However, he does not agree, and claims that you cannot know that $6$ would equal $8$ if $4$ were to equal $5$. Is the statement in the title correct?

$$4=5$$
$$4-1=5-1$$
$$2(4-1)=2(5-1)$$
