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Problem
Calculating 15% or 20% tips at restaurants requires mental math under pressure.
Solution
10% = move decimal left. 20% = double that. 15% = 10% + half of 10%.
Benefit
Calculate common tip percentages in 3 seconds without calculator.
Example
Bill: $84.50
10% = $8.45
20% tip = $8.45 × 2 = $16.90
15% tip = $8.45 + $4.23 = $12.68
Bill: $42.00
10% = $4.20
15% tip = $4.20 + $2.10 = $6.30