Uploaded July 2026 | Updated September 2026, 2 weeks ago
Ron White, 2x USA Memory Champion finds a stranger on the street and teaches him this math hack to calculate percentages fast in your head.
One of the fastest ways to calculate percentages mentally is to divide both the percentage and the number by 10, then multiply the results. This works because you're reducing both numbers by the same factor, so the answer stays the same.
The trick is especially useful when the percentage ends in 0, since you avoid decimals and the math becomes very simple. For example, to find 30% of 150, divide both numbers by 10 to get 3 × 15 = 45. Likewise, 40% of 250 becomes 4 × 25 = 100.
These problems can now be solved with easy multiplication instead of thinking about percentages.
The same idea also works when the percentage doesn't end in zero—you'll just be multiplying decimals instead of whole numbers.
For example, 25% of 80 becomes 2.5 × 8 = 20, and 35% of 240 becomes 3.5 × 24 = 84. While decimals make the arithmetic a little less convenient, it's still often easier than converting the percentage to a decimal or setting up a proportion.
Once you get used to it, this becomes a powerful mental math shortcut for almost any percentage problem.
Ron White, 2x USA Memory Champion finds a stranger on the street and teaches him this math hack to calculate percentages fast in your head.
One of the fastest ways to calculate percentages mentally is to divide both the percentage and the number by 10, then multiply the results. This works because you're reducing both numbers by the same factor, so the answer stays the same.
The trick is especially useful when the percentage ends in 0, since you avoid decimals and the math becomes very simple. For example, to find 30% of 150, divide both numbers by 10 to get 3 × 15 = 45. Likewise, 40% of 250 becomes 4 × 25 = 100.
These problems can now be solved with easy multiplication instead of thinking about percentages.
The same idea also works when the percentage doesn't end in zero—you'll just be multiplying decimals instead of whole numbers.
For example, 25% of 80 becomes 2.5 × 8 = 20, and 35% of 240 becomes 3.5 × 24 = 84. While decimals make the arithmetic a little less convenient, it's still often easier than converting the percentage to a decimal or setting up a proportion.
Once you get used to it, this becomes a powerful mental math shortcut for almost any percentage problem.
