My one-sentence advice to those about to write an exam: Number your carbons.
Now go write your exam.
If you need reasons, here they are. There’s two.
First reason: if you number all the carbons in your starting compound(s), it helps you double check that you didn’t miss any in your product. Under the high stress atmosphere of an exam, it’s easy to make little mental mistakes like drawing a 4 carbon chain as a 3 carbon chain and so on. This is a quick protocol that will help to prevent that.
Second reason: When drawing products of a given reaction, numbering all the carbons helps you avoid getting lost. It’s a shame to watch students recognize where all the electron-pushing arrows are supposed to go only to draw their product incorrectly. Figuring out where the electrons are supposed to go is the hard part. Drawing the new product should be a slam dunk. Numbering really helps with this.
2 Quick examples of what I mean:
Note that when I say “number” I don’t mean do it in the “proper”, IUPAC way. I just mean label them in some way that makes each carbon unique so you know where to stitch the pieces together.
Theseus needed a ton of skill to hunt down and defeat the minotaur in the labyrinth. But in the end it was that 99 cent ball of twine that saved his butt by helping him find his way out.
Numbering the carbons is your ball of twine.