How do you compute a common tip or discount, like 15% or 30%?

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.