**Important Concepts and Formulas**

1. The factor of a given number divides
the number exactly.

2. Every number greater than 1 has at
least two factors.

3. A multiple of a number is the product
of that number by any other whole number.

4. Every number has infinite number of
multiples.

5. Every number is its smallest
multiple.

6. Every number other than 1, whose only
factors are 1 and the number itself is called a prime number.

8. The number 1 is neither a prime nor a
composite number.

9. The only even prime number is 2, all
other prime numbers are odd numbers.

10. There are 25 prime numbers between 1
and 100.

11. All the multiples of 2 are called
even numbers.

12. The numbers which are not multiples
of 2 are called odd numbers.

13. A number is completely divisible by 2
if it ends with 0, 2, 4, 6 or 8.

14. A number is completely divisible by 3
if the sum of its digits is a multiple of 3.

15. If a number is divisible by both 2
and 3, then it is divisible by 6.

16. A number is completely divisible by 4
if the number formed by its last two digits is divisible by 4.

17. A number is divisible by 5 if it has
0 or 5 in its ones place.

18. If the sum of digits of a number is a
multiple of 9, then the number is divisible by 9.

19. A number is divisible by 10 if it has
0 in its ones place.

20. A number is completely divisible by 8
if the number formed by its last three digits is divisible by 8.