30
How about: 14+14 = 28 because 14 times 1 = 14
666
They are: 11 13 17 19 31 41 61 and 71 which makes 8 of them
14 and 35
To show very large or very small numbers, without writing out all the digits. To make it easy to compare such numbers, without having to count all the digits.
Anything having to do with adding or subtracting unlike fractions.
9
The whole numbers having 9 as a factor are all multiples of 9. The first twelve of these numbers are 9, 18, 27, 36, 45, 54, 63, 72, 81, 90, 99 and 108. NOTE : the sum of the digits any number which is a multiple of 9 add up to 9 or a multiple of 9. For example 3501 is divisible by 9 as 3+5+0+1 = 9
5 digit telephone numbers having at least one of their digits repeated is = total possible 5 digit telephone numbers - 5 digit telephone numbers without any digit being repeated. =(10*10*10*10*10)-(10*9*8*7*6) =100000-30240 =69760
The number nine is a composite number (3 x 3 = 9) and it is a square number (32 = 9). It is a divisor of any number that has digits having the sum of nine or a multiple of 9 (For example, the digits in 367,218 add up to 27, which is a multiple of 9, and the digits in 27 also add up to 9)
In representing large numbers, English texts use commas (or spaces) to separate each group of three digits. This is almost always done for numbers of six or more digits, and optionally for five (or even four) digits. This makes it easier to read the number without having to count how many place holders there are in the number as a whole.
1234567
987,654