Some Emergency Formulas for Mathematics

1. A number is divisible by ‘2’ if it ends in zero or in a digit which is a multiple of ‘2’ i.e. 2, 4, 6, 8.

2. A number is divisible by ‘3’, if the sum of the digits is divisible by ‘3’.

3. A number is divisible by ‘4’ if the number formed by the last two digits, i.e. tens and units are divisible by 4.

4. A number is divisible by ‘5’ if it ends in zero or 5

5. A number is divisible by ‘6’ if it divisible by ‘2’ as well as by ‘3’.

6. A number is divisible by ‘8’ if the number formed by the last three digits, i.e, hundreds tens and units is divisible by ‘8’.

7. A number is divisible by ‘9’ if the sum of its digit is divisible by ‘9’

8. A number is divisible by ‘10’ if it ends in zero.

9. A number is divisible by ‘11’ if the difference between the sums of the digits in the even and odd places is zero or a multiple of ‘11’.

10. The square of an odd number is always odd.

11. A number is said to be a prime number if it is divisible only by itself and unity. Ex. 1, 2, 3, 5,7,11,13 etc.

12. The sum of two odd numbers is always even.

13. The difference of two odd numbers is always even.

14. The sum or difference of two even numbers is always even.

15. The product of two odd numbers is always odd.

16. The product of two even numbers is always even.