Finding 10% and Building From There

Mental Math · Axiom Academy

LESSON Finding 10% and Building From There Master the foundational mental math technique of finding 10% by moving the decimal, then building other percentages from it 1. The 10% Trick: Move the Decimal Why does this work? Because 10% means "10 per hundred" or 1/10 of the number. Dividing by 10 is the same as moving the decimal left! 10% of 1,450: 1450.0 → 145.0 = 145 Once you know 10%, you can quickly find many other percentages by simple multiplication, division, or addition. Let's see how! 3. Real-World Example: Restaurant Tips Let's break this down using our 10% foundation: Step 1: Find 10% of 87: Move decimal → 8.70 Step 2: Find 5% (half of 10%): 8.70 ÷ 2 = 4.35 Step 3: Add them together: 8.70 + 4.35 = 13.05 15% tip = 13.05 (or round to 13 for convenience!) 4. Real-World Example: Shopping Discounts 25% might seem tricky, but with our technique, it's just combining pieces we know! Step 4: 25% = 20% + 5% = 32 + 8 = 40 savings! Since 25% = 1/4, you can also divide by 4: Both methods work! Use whichever feels more natural to you. 5. Advanced Applications: Any Percentage Now that you've mastered the basics, you can tackle any percentage by breaking it down into pieces of 10%, 5%, and 1%: 1% of 75: 0.75 (move decimal two places) 18% = 10% + 8%: 7.50 + 6.00 = 13.50

This is the written version of the interactive lesson above. See the full Mental Math course.