6 Maths Tricks You Probably Don’t Know

1. Square
   To square a 2 digit number ending in 5:

   [Example]

   25² = ?

   • Mulitply the first digit by itself + 1
     2 x (2+1)
   • Put 25 at the end of the value
     (2 x 3) & 25

   Answer = 625

2. Subtracting from 1000
   To subtract a large number from 1000:

   [Example 1]

   1000 – 649 = ?

   • Subtract all but the last number from 9
     9-6 = 3
     9-4 = 5
   • then subtract the last number from 10:
     10-9 = 1

   Answer = 351

   [Example 2]

   10000 – 6649 = ?

   • Subtract all but the last number from 9
     9-6 = 3
     9-6 = 3
     9-4 = 5
   • Subtract the last number from 10:
     10-9 = 1

   Answer = 3351

3. Dividing by 5
   To divide a large number by five:

   [Example 1]

   195 / 5 = ?

   • Multiply the number by 2
     195 x 2 = 390
   • Shift the decimal point one space to the left
     39.0, or 39

   Answer = 39

   [Example 2]

   2978 / 5 = ?

   • 2978 x 2 = 5956
   • 595.6

   Answer = 595.6

   If you find it easier like me to just divide by 5, just go ahead.

4. Multiplying by 9
   To multiply any number between 1 to 9 by 9:

   [Example]

   8 x 9 = ?

   • Hold up both hands in front of you, with fingers extended.
   • Drop the finger that corresponds to the number you are multiplying (e.g. for 8 x 9, drop your 8th finger).
   • Count the fingers before the dropped finger (which is 7) — that’s the first digit.
   • Count the fingers after (which is 2). Combined both digits.

   Answer = 72

5. Multiplying by 11

   [Example 1]

   42 x 11 = ?

   • With a given number like 42, leave a space between between the digits: 4 2
   • Add the two digits and put the answer right into the middle space created: 4 (4+2) 2

   Answer = 462

   [Example 2]

   88 x 11 = ?

   • Leave a space between between the digits: 8 8
   • Add the two digits and put the answer right into the middle space created: 8 (8+8) 8
   • If the numbers in the middle add up to a 2 digit number, put in the second digit into the middle space: 8 (6) 8
   • Add 1 to the the first number: (8+1) 6 8

   Answer = 968

6. Multiplying big numbers

   [Example]

   32 x 125 = ?

   • Break it down when one of the numbers is even
     32 x 125 is the same as:
     16 x 250 is the same as:
     8 x 500 is the same as:
     4 x 1000 = 4000

   • Answer = 4000

Inspiration: Listverse.com

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