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Question
What are the two anchor percentages you build everything else from?
Answer
10% (move the decimal one place left) and 1% (move it two places) — every other percentage is a combination of these.
- 10% of 340 = 34.0 (decimal shifts one place).
- 1% of 340 = 3.40 (two places).
From those, 5% is half of 10% (17), 20% is double (68), 15% is 10% + 5% (51), 2% is double 1% (6.8). Almost any percentage is a quick sum of 10%s, 5%s, and 1%s.
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Question
How do you compute a common tip or discount, like 15% or 30%?
Answer
Break it into 10% and 5% (or ×3 of 10%) and add or subtract.
- 15% tip on $60 → 10% = 6, 5% = 3 → 6 + 3 = $9.
- 30% off $80 → 10% = 8, ×3 = 24 → 80 − 24 = $56.
- 20% tip on $45 → 10% = 4.5, ×2 = $9.
A discount is the same percentage math, then subtracted from the original; a tip is added. Compute the percentage first, adjust after.
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