A dollar can be made into small change in 293 ways:

You can remember the value of Pi (3.1415926) by counting each word's letters in "May I have a large container of coffee?" 

Multiplication and succession: 
1 × 8 + 1 = 9
12 × 8 + 2 = 98
123 × 8 + 3 = 987
and so on. 
Submitted by: Moi, Canada.Submitted by: Moi, Canada. 

142857 is a cyclic number, i.e., its digits are rotated around when multiplied by any number from 1 to 6. Like this: 
142857 × 1 = 142857 
142857 × 5 = 7 14285
142857 × 4 = 57 1428
142857 × 6 = 857 142
142857 × 2 = 2857 14
142857 × 3 = 42857 1
Submitted by: Moi, Canada.Submitted by: Moi, Canada. 

1089 multiplied by 9 gives an exact reverse: 9801. 

From 0 to 1,000, the letter "A" only appears in 1,000 ("one thousand"). 

2 is called the "oddest" Even Prime-number. 2 is a unique Even Prime because all Even-numbers are divisible by 2. But any number apart from 2 that is divisible by 2, is not a Prime. 

1 × 9 + 2 = 11
12 × 9 + 3 = 111
123 × 9 + 4 = 1111
... and so on. 
Submitted by: RobSubmitted by: Rob 

40 when written "forty" is the only number with letters in alphabetical order, while "one" is the only one with letters in reverse order. 
Submitted by: RobSubmitted by: Rob 

1 googol = 10100;
1 googolplex = 10googol = 1010100 . 
111 111 111 x 111 111 111 = 12345678 9 87654321 

Pi (3.14159...) is a number that cannot be written as a fraction. 
Submitted by: Tiff - Witherbee, United StatesSubmitted by: Tiff - Witherbee, United States 

The opposite sides of a dice cube always add up to seven. 
Submitted by: Ramesh - Pune, IndiaSubmitted by: Ramesh - Pune, India 

21978 when multiplied by 4 is the same number with digits in reverse order: 21978 x 4 = 87912 

If you add up the numbers 1-100 consecutively (1+2+3+4+5...) the total is 5050. 

The billionth digit of Pi is 9. 

1 and 2 are the only numbers where they are the values of the numbers of factors they have. 

2 and 5 are the only primes that end in 2 or 5. 

The largest prime number is 9,808,358 digits long; more than the number of atoms in the universe. 

The digits to the right of the Pi's (3.141...) decimal point can keep going forever, and there is no pattern to these digits at all. 

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