The following statement seems too obvious to prove or even to mention:
That the area of a rectangle with side lengths $a$ and $b$ equals the area of a rectangle with side lengths $(ab)/c$ and $c$ for arbitrary $c>0$, i.e.
$$ a \times b = ((ab)/c) \times c$$
(Note that $(ab)/c$ is a shorthand for $a/(c/b)$.)
I wonder how this statement would have been formulated by Euclid and how he would have proved it (as a geometrical statement with geometrical methods).
I guess there is no proposition in Euclid's Elements that says exactly this, but maybe there is one which says essentially the same - or there is an easy proof starting from theorems already proved by Euclid.
This is how how I would formulate the statement:
Consider two figures constructed like this:
and like that:
Then they are equal (in size).