What is P% of any number?

The simplest percentage question

“What is X% of Y?” is the most common percentage calculation in everyday life. Three ways to compute it, all giving the same answer:

• Decimal method: convert the percent to a decimal, then multiply. 15% of 80 → 0.15 × 80 = 12.
• Fraction method: multiply, then divide. 15% of 80 → 15 × 80 ÷ 100 = 12.
• Per-1 method: divide by 100 to get the value of 1%, then multiply. 80 ÷ 100 = 0.8, then 0.8 × 15 = 12.

Useful mental shortcuts

• 10% of anything: move the decimal one place left. 10% of 250 = 25.
• 1% of anything: move the decimal two places left. 1% of 250 = 2.5.
• 5% of anything: half of 10%. 5% of 250 = 12.5.
• 50% of anything: half. 50% of 250 = 125.
• 25% of anything: quarter. 25% of 250 = 62.5.
• 15% of anything: 10% + half of 10%. 15% of 250 = 25 + 12.5 = 37.5.
• 20% of anything: double the 10%. 20% of 250 = 50.

Common applications

Sales tax (5–10%), restaurant tips (15–20%), pay raises (3–7%), commission rates (1–10%), interest (mortgage rates 3–8%), discounts (10–80%). All of these are just “what is X% of Y”.