Stage 17 of 20
Percentage Shortcuts
Find 10%, 5%, 20%, and 50% of friendly amounts without a calculator — useful for tips, sales, and school word problems.
Start from 10% and 50%
10% of a number is the number ÷ 10. 50% is half. 5% is half of 10%. 20% is double 10%. Build other percents from these bricks.
Why this mental math skill matters
Percentages show up in shopping, tips, sports stats, and school word problems. Building from 10% and 50% keeps the work mental and practical.
Worked examples
Study each example slowly. Say the steps aloud — verbal rehearsal strengthens mental arithmetic memory.
10% of 80
- Move one place value: 80 ÷ 10.
- 8.
Answer: 8
20% of 45
- 10% of 45 = 4.5.
- Double it → 9.
Answer: 9
15% of 60
- 10% = 6; 5% = 3.
- 6 + 3 = 9.
Answer: 9
Tips for success
- 10% = divide by 10.
- 5% = half of 10%.
- 20% = double 10%; 15% = 10% + 5%.
Common mistakes to avoid
- Mixing up percent of a number with percentage points.
- Forgetting to halve/double after finding 10%.
How to practice this stage
Find 10% first on every problem, then build 5%, 20%, or 15% from that brick. Use friendly whole numbers while learning.
Interactive practice
Stage 17 Drill
Calculate simple percentages mentally.
Solve
—
FAQ: Percentage Shortcuts
How do you calculate 15% mentally?
Find 10%, find 5% (half of that), then add them together.
Why start with 10%?
Because many common percentages are simple combinations of 10% and 5% or 50%.