Find percentages and ratios
Percentages are just fractions out of 100. The word literally translates to "per cent" (for every hundred).
There is a fundamental algebraic property of multiplication: A × B = B × A.
Because calculating a percentage is just multiplication, this means that x% of y is always exactly equal to y% of x. This is a legendary mental math trick.
Imagine someone asks you to calculate 4% of 75 in your head. For most people, that is very difficult math.
But if you use the reversibility trick, you just flip the numbers: What is 75% of 4?
Suddenly, the math is trivial. 75% is just three-quarters. Three-quarters of 4 is 3. Therefore, 4% of 75 is also exactly 3.
When calculating Percentage Change (like a stock going up or down), always remember that the formula divides the absolute difference by the original number, not the new number.
This creates an asymmetrical trap: A stock dropping from ₹100 to ₹50 is a 50% loss. But for that same stock to go from ₹50 back up to ₹100, it requires a 100% gain! Losing money requires much higher percentage gains just to break even.
Using the reversibility trick, what is the easiest way to calculate 18% of 50 in your head?
