Most of the numbered "rules" that show up in marketing searches are shorthand for one real idea each, not laws, and a few are worth knowing while others are closer to trivia. Here's what each one actually claims, and how much weight to actually put on it.
The 40-40-20 rule
Originally about direct mail, now applied to campaigns generally: your audience matters roughly 40% of your result, your offer matters another 40%, and your creative, the ad or copy itself, accounts for the remaining 20%. The practical takeaway holds up well: businesses spend most of their effort polishing the ad and the least effort figuring out who they're actually targeting and what they're offering, backwards from where the leverage actually is.
The 3-7-27 rule
A social media specific claim that someone typically needs to see a post 3 times before noticing it, 7 times before it registers, and 27 exposures before they take action. The exact numbers are more folklore than research, but the underlying point, that one impression rarely converts anyone, is accurate and worth remembering when a single ad doesn't perform and gets pulled too early.
The 70/20/10 rule
A content allocation guideline: 70% of content should be proven, low-risk formats you know work, 20% should build on what's working with some variation, and 10% should be genuinely experimental. A reasonable framework for avoiding two failure modes at once, playing it too safe forever, or gambling the whole content strategy on unproven ideas.
The 80/20 rule
The Pareto principle applied to marketing: roughly 80% of results come from 20% of effort, channels, or customers. In funnel terms, this usually means a small number of traffic sources or content pieces drive most conversions, worth actually checking your own numbers for rather than assuming evenly distributed effort is paying off evenly.
The 60/40 and 50/30/20 rules
Both are budget-split guidelines, one for general marketing spend allocation, one specifically for social media content mix. Useful as a starting point if you have no existing data to work from, but a business with real conversion data should let that data override any generic ratio.
The one that actually matters most
Not a numbered rule at all: know where your funnel actually leaks before optimizing anything else. Every rule above is a reasonable default in the absence of your own data. None of them beat looking at your actual funnel's step-by-step drop-off and fixing the specific stage losing the most people, which is usually the middle, not the ad and not the checkout page.