Numeracy

Basic Mental Math Hacks




Mental arithmetic is a valuable skill for tackling everyday calculations without tools. These simple tricks can make calculations faster and less intimidating, whether you're estimating prices or solving workplace problems.

Multiplying Numbers by 10, 100, and 1000

  1. Multiply by 10:
  2. Move digits one place to the left.
  3. Add a zero if needed.

  4. Examples:

    • ( 24 * 10 = 240 )
    • ( 3.56 * 10 = 35.6 )

  5. Multiply by 100:

  6. Move digits two places to the left.

  7. Examples:

    • ( 845 * 100 = 84,500 )
    • ( 37.64 * 100 = 3764 )

  8. Multiply by 1000:

  9. Move digits three places to the left.
  10. Examples:
    • ( 23.476 * 1000 = 23,476 )

For Multiples of 10, 100, or 1000:

  • Multiply the base number first, then shift the digits.
  • Example: ( 25 * 5000 ) Multiply ( 25 * 5 = 125 ), then move three places ( 125,000 ).

Dividing Numbers by 10, 100, and 1000

  1. Divide by 10:
  2. Move digits one place to the right or move the decimal point one place left.

  3. Example: ( 785 \div 10 = 78.5 ).

  4. Divide by 100:

  5. Move digits two places to the right.

  6. Example: ( 56 \div 100 = 0.56 ).

  7. Divide by 1000:

  8. Move digits three places to the right.
  9. Example: ( 450 \div 1000 = 0.45 ).

For Multiples of 10, 100, or 1000:

  • Divide by the base first, then adjust digits.
  • Example: ( 56 \div 7000 ) Divide ( 56 \div 7 = 8 ), then move three places ( 0.008 ).

Adding and Subtracting Made Easy?

Break numbers into manageable parts:
- Example 1: ( 352 - 13 ):
- Subtract ( 10 ), then ( 3 ): ( 352 - 10 = 342; 342 - 3 = 339 ).

  • Example 2: ( 737 + 96 ):

  • Add ( 100 ), then subtract ( 4 ): ( 737 + 100 = 837; 837 - 4 = 833 ).

  • Example 3: ( 5372 - 985 ):

  • Subtract ( 1000 ), then add ( 15 ): ( 5372 - 1000 = 4372; 4372 + 15 = 4387 ).

Mental Hacks for Multiplication?

Use approximation and breakdown:
1. Example: ( 97 * 7 ):
- Method 1: Think of ( 97 ) as ( (100 - 3) ):
- ( (7 * 100) - (7 * 3) = 700 - 21 = 679 ).
- Method 2: Start with ( 7 * 100 = 700 ), then subtract ( 7 * 3 = 21 ):
- ( 700 - 21 = 679 ).

Percentages and Money Tricks

  1. Round Prices:

  2. Example: Treat ( $24.99 ) as ( $25 ) for quick calculations.

  3. Reversible Percentages:

  4. ( 16\% ) of ( 25 ) = ( 25\% ) of ( 16 ) Easier to calculate ( 4 ).

Summing it up

  • Mental arithmetic is about breaking problems into simpler parts (chunking).
  • Focus on moving digits or decimal points instead of memorizing answers.
  • Practice these tricks, and they'll become second nature!
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