You’re at a store. 35% off on a 4,200 item. The cashier is waiting. Your phone is in your pocket. Here’s how to get the answer in your head.

The Core Tricks

1. Start with 10%

Move the decimal point one place left. 10% of anything is trivial.

• 10% of 4,200 = 420
• 10% of 875 = 87.5
• 10% of 12,000 = 1,200

This is your building block for everything else.

2. Build from 10%

• 5% = half of 10%
• 20% = double 10%
• 25% = quarter of the number (divide by 4)
• 15% = 10% + 5%
• 30% = 3 x 10%
• 35% = 30% + 5%

Back to our example: 35% of 4,200

• 10% = 420
• 30% = 1,260
• 5% = 210
• 35% = 1,260 + 210 = 1,470

Price after discount: 4,200 - 1,470 = 2,730

3. The Flip Trick

8% of 50 is the same as 50% of 8 = 4.

Always calculate whichever direction is easier. 7% of 200? Flip it: 200% of 7 = 14.

This works because multiplication is commutative: A% of B = B% of A.

4. The 1% Method

For weird percentages, find 1% first.

17% of 6,500:

• 1% = 65
• 17% = 65 x 17 = 65 x 10 + 65 x 7 = 650 + 455 = 1,105

5. Percentage Change Shortcut

To find percentage change: find the difference, then see what fraction it is of the original.

Price went from 800 to 920.

• Difference: 120
• 120 out of 800: that’s 12/80 = 15/100 = 15% increase

Common Scenarios

Restaurant tipping

For a 15% tip on a 2,400 bill:

• 10% = 240
• 5% = 120
• Total tip = 360

Sale shopping

40% off a 3,500 item:

• 40% = 4 x 350 = 1,400
• You pay: 3,500 - 1,400 = 2,100

Or think of it as paying 60%: 6 x 350 = 2,100

Tax calculations

18% GST on 5,000:

• 10% = 500
• 8% = 4 x 100 = 400
• GST = 900. Total = 5,900.

The Two-Second Rule

For any percentage calculation, ask: “What’s the easiest way to break this into chunks of 10%, 5%, or 1%?” Almost any percentage decomposes into a quick sum of these building blocks.

Practice for a week, and you’ll never reach for the calculator app for basic percentages again.