# Playing with Numbers

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Numbers can be written in general form. Thus a two digit number ab will be written as ab = 10a + b and a three-digit number abc will be written as abc = 100a + 10b + c.

Sum of a number and a number after reversing its digits will always be a multiple of 11.
- A number is divisible by 2 if it has any of the digits 0, 2, 4, 6 or 8 at its ones place.
- A number is divisible by 3 if the sum of its digits is a multiple of 3.
- A number is divisible by 5 if it has either 0 or 5 at its ones place.
- A number is divisible by 10 if it has 0 at its ones place.
- A number is divisible by 9 if the sum of its digits is a multiple of 9.
- A number will be divisible by 11 if the difference between the sum of digits at its odd places and that of digits at the even places is divisible by 11.
- A number is divisible by another number if the quotient is a whole number and the remainder is zero.
- A factor of a number is an exact divisor of that number, i.e., the factor leaves remainder 0 when it divides the number.
- A multiple of a number is the product of that number with any natural number.

The first digit of a number from right side is called its ones place.

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