Divisibility

Question 1

How to check if divisible by 2

Answer 1

Is it even?

Question 2

How to check if divisible by 3

Answer 2

Add all digits together, see if the sum is divisible by 3 (repeat as much as necessary)
E.G 12345 = 1+2+3+4+5=15, which is divisible by 3.

Question 3

How to check if divisible by 5

Answer 3

Does it end in 5 or 0?

Question 4

How to check if divisible by 7

Answer 4

Double the last digit, subtract it from the remainder of the digits (repeat as much as necessary)
E.G 4767 = 476 - 14 = 462 | 46 - 4 = 42 (6*7) | 4 - 4 = 0 (0 is divisible by 7)

Question 5

How to check if divisible by 11

Answer 5

Add and subtract each digit in the number by an alternating pattern, repeat, until you can tell if the number is divisible by 11 (start from 0)
E.G: 3949 = 0+3-9+4-9= -11 (is divisible by 11)

Question 6

How to tell if a number is prime?

Answer 6

Check divisibility by approximate square root. E.G 250 approximate square root is 15. Check if it is divisible by any prime numbers below that - if not, then it is prime