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Showing 30 results for the selected filters.
Clear filtersSquaring numbers like 48 or 52 requires long multiplication.
Use (50-a)² = 2500 - 100a + a² or (50+a)² = 2500 + 100a + a²
Converts difficult squares into simple arithmetic around 2500.
Testing divisibility by 4 for large numbers requires division.
Check only the last two digits. If they form a number divisible by 4, the whole number is.
Instantly test any number for divisibility by 4.
Checking divisibility by 6 requires checking both 2 and 3 separately.
Number must be even AND sum of digits divisible by 3.
Combines two simple rules for quick divisibility check.
Multiplying by 25 requires complex long multiplication.
Divide by 4 and multiply by 100 (or multiply by 100 and divide by 4).
Converts multiplication into simple division by 4.
Figuring out what day of the week a date falls on requires a calendar.
Use Zeller's congruence simplified: Learn anchor days for each month and count forward/backward.
Impress others by calculating any date's day of week mentally.
Calculating how long it takes for investment to double requires complex formulas.
Divide 72 by the interest rate to get approximate years to double.
Quick investment doubling time calculation without formulas or calculators.
Finding cube roots mentally is extremely difficult.
Memorize cubes 1-10, then find which two cubes the number falls between.
Estimate cube roots within ±0.5 by knowing 10 reference points.
Calculating percentage change between two values is confusing and error-prone.
((New - Old) / Old) × 100. Positive = increase, negative = decrease.
Universal formula for all percentage change calculations.
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