The Law of Diminishing Returns: The rate at which the increase in cost decreases is lower than the rate at which the increase in benefit decreases.
As teh interwebz says, "Few understand this."
One is a certain amount. Two is twice as much as one. Three is only 50% more than two. Four is only 25% more than three. Five is only 20% more than four.
Are you starting to notice a pattern? This can help you get the "most bang for your buck."
The 80/20 Rule
A few centuries ago, an Italian accountant noticed that 20% of the population owned 80% of the wealth. Scrutinizing that 20% further, he saw that 20% of the 20% had 80% of the 80%.
This has since been found to hold true in many areas of life.
You can apply this to exercise; lifting one day a week should give you about 80% of the benefit of lifting 5 days a week. That can free up your schedule tremendously.
But Wait, There's More!
The 80/20 rule is a specific application; the Law of Diminishing Returns may manifest differently in other situations.
Let's say doing something six times gives you the maximum benefit you could get from doing that thing.
Doing it once might get you 25% of doing it 6 times. Twice might get you up to 50% of 6 times. Three times = 70%, four times = 85%, five times = 95%.
In that case, doing it 6 times would give 100%, no? Actually, no. Six times is 96%, 7 times is 97%, 8 times is 98%, 9 times is 99%.
So, 10 times is 100% of doing it six times?! Still wrong! Ten times is 99.1%
Bake Your Noodle
Eleven times is 99.11%. Gosh, this is starting to seem like Xeno's Paradox.
The closer you get to perfection, the more inefficient the effort becomes. Appreciating the Law of Diminishing Returns can save you wasted resources in real life.
Of course, if you sit down to eat a pie, the first two bites won't reduce the pie by half; if you eat 100% of the pie, you've eaten 100% of the pie.
The Law of Diminishing Returns and Xeno's Paradox don't apply to absolutely everything!