### Multiplying by 6

If you multiply 6 by an even number, the answer will end with the same digit. The number in the tens place will be half of the number in the ones place.

**Example**: 6 x 4 = 24

### The Answer Is 2

- Think of a number.
- Multiply it by 3.
- Add 6.
- Divide this number by 3.
- Subtract the number from Step 1 from the answer in Step 4.

The answer is 2.

### Same Three Digit Number

- Think of any three digit number in which each of the digits is the same. Examples include 333, 666, 777, 999.
- Add up the digits.
- Divide the three digit number by the answer in Step 2.

The answer is 37.

### Six Digits Become Three

- Take any three digit number and write it twice to make a six digit number. Examples include 371371 or 552552.
- Divide the number by 7.
- Divide it by 11.
- Divide it by 13. (The order in which you do the division is unimportant.)

The answer is the three digit number

**Examples**: 371371 gives you 371 or 552552 gives you 552.

- A related trick is to take any three digit number.
- Multiply it by 7, 11, and 13.

The result will be a six digit number that repeats the three digit number.

**Example**: 456 becomes 456456.

### The 11 Rule

This is a quick way to multiply two digit numbers by 11 in your head.

- Separate the two digits in your mind.
- Add the two digits together.
- Place the number from Step 2 between the two digits. If the number from Step 2 is greater than 9, put the ones digit in the space and carry the tens digit.

**Examples**: 72 x 11 = 792

57 x 11 = 5 _ 7, but 5 + 7 = 12, so put 2 in the space and add the 1 to the 5 to get 627

### 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 gives 3.141592

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

- Select a number from 1 to 6.
- Multiply the number by 9.
- Multiply it by 111.
- Multiply it by 1001.
- 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.