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- Multiplying by 9 – Use your fingers! For 9 × 7, fold your
7th finger; the fingers before = 6, after = 3 → 63.
- Square Numbers Ending in 5 – Multiply the first digit(s) by next
higher number, append 25. Example: 25² → 2×3=6 → 625.
- Quick Percentage Calculation – 10% of any number = divide by 10. Then
halve/double as needed for 5%, 20%, etc.
- Sum of Consecutive Numbers – Use formula: n(n+1)/2. Example:
1+2+…+100 = 5050.
- Divisibility Rules – 3: sum of digits divisible by 3; 4:
last two digits divisible by 4; 11: difference of alternate digits
divisible by 11.
- Multiplying by 11 – Add adjacent digits. Example: 23×11 →
2(2+3)3 → 253.
- Percentage Increase Shortcut – New value = original × (1 + %/100).
- Finding LCM & HCF – Use prime factorization; multiply
highest powers for LCM, lowest for HCF.
- Fast Fractions to Decimals – Divide numerator by denominator using
long division or memorization for common fractions.
- Mental Math – Break numbers into smaller parts for addition/subtraction.
Example: 76+59 → 76+50=126 → 126+9=135.
