Simple and useful math tricks

 1. Memorizing Pi

To remember the first seven digits of pi, count the number of letters in each word of the sentence:

"How I wish I could calculate pi."

This becomes 3.141592.


2. Contains the Digits 1, 2, 4, 5, 7, 8

  1. Select a number from 1 to 6.
  2. Multiply the number by 9.
  3. Multiply it by 111.
  4. Multiply it by 1001.
  5. Divide the answer by 7.

The number will contain the digits 1, 2, 4, 5, 7, and 8. 

Example: The number 6 yields the answer 714285.


3.  Multiply Large Numbers in Your Head

To easily multiply two double-digit numbers, use their distance from 100 to simplify the math:


Subtract each number from 100.

Add these values together.

100 minus this number is the first part of the answer.

Multiply the digits from Step 1 to get the second part of the answer.


4. Super Simple Divisibility Rules

You've got 210 pieces of pizza and want to know whether or not you can split them evenly within your group. Rather than whip out the calculator, use these simple shortcuts to do the math in your head:


Divisible by 2 if the last digit is a multiple of 2 (210).

Divisible by 3 if the sum of the digits is divisible by 3 (522 because the digits add up to 9, which is divisible by 3).

Divisible by 4 if the last two digits are divisible by 4 (2540 because 40 is divisible by 4).

Divisible by 5 if the last digit is 0 or 5 (9905)


5. Finger Multiplication Tables

Everyone knows you can count on your fingers. Did you realize you can use them for multiplication? A simple way to do the "9" multiplication table is to place both hands in front of you with fingers and thumbs extended. To multiply 9 by a number, fold down that number finger, counting from the left.


Examples: To multiply 9 by 5, fold down the fifth finger from the left. Count fingers on either side of the "fold" to get the answer. In this case, the answer is 45.


To multiply 9 times 6, fold down the sixth finger, giving an answer of 54.

Divisible by 6 if it passes the rules for both 2 and 3 (408).

Divisible by 9 if the sum of the digits is divisible by 9 (6390 since 6 + 3 + 9 + 0 = 18, which is divisible by 9).

Divisible by 10 if the number ends in a 0 (8910).

Divisible by 12 if the rules for divisibility by 3 and 4 apply.

Example: The 210 slices of pizza may be evenly distributed into groups of 2, 3, 5, 6, 10


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