Fast Percentage Tricks: Calculating 10%, 15%, and 20% in Your Head

Percentages have a reputation for being tricky, but they are actually one of the most practical everyday math skills your child will ever learn. Whether calculating a discount at a shoe store or figuring out sales tax, you do not need a smartphone calculator if you know the 10% building block strategy.

The Anchor: Finding 10%

Every fast mental percentage starts with 10%. To find 10% of any number, simply move the decimal point one place to the left:

• 10% of $60 = $6.00
• 10% of $45 = $4.50
• 10% of $120 = $12.00

How to Find 5%

5% is simply half of 10%. Once you have 10%, divide it by 2:

• 10% of $40 is $4. So 5% is half of $4 = $2.
• 10% of $80 is $8. So 5% is half of $8 = $4.

How to Find 15% (The Tip Trick)

To find 15%, add 10% and 5% together:

How to Find 20%

20% is simply double 10%:

• 10% of $70 is $7. Double $7 = $14.
• 10% of $35 is $3.50. Double $3.50 = $7.00.

Teacher Tip: The Reversible Percentage Secret

Here is a secret that even many adults do not know: x% of y is always equal to y% of x.

If someone asks you to find 16% of 50, that sounds difficult. But reverse it: 50% of 16! 50% just means half. Half of 16 is 8. Therefore, 16% of 50 is also 8!