Prime Numbers and Divisibility Flashcards
Prime #s between 0 and 10
2, 3, 5 ,7
Prime #s between 10 and 20
11, 13, 17, 19
Prime #s between 20 and 30
23, 29
Prime #s between 30 and 40
31, 37
Prime #s between 40 and 50
41, 43, 47
Prime #s between 50 and 60
53, 59
Prime #s between 60 and 70
61, 67
Prime #s between 70 and 80
71, 73, 79
Prime #s between 80 and 90
83, 89
Prime #s between 90 and 100
97
Prime #s between 100 and 110
101, 103, 107, 109
Divisibility by 3
The SUM of the integers DIGITS must be divisible by 3
Divisibility by 4
The last TWO DIGITS of the number must be divisible by 4
Divisibility by 5
If the integers ends in 0 or 5
Divisibility by 6
Integer must be divisible by BOTH 2 and 3
Divisibility by 8
The last THREE Digits of the integer must be divisible by 8 OR divisible by 2 THREE TIMES
Divisibility by 9
The SUM of the integers DIGITS must be divisible by 9
Divisibility by 10
the integer ends in 0
Factor Foundation Rule: if a is divisible by ‘b’ and ‘b’ is divisible by ‘c’ -> then ‘a’ is divisible by ‘c’ as well!
Example: 12 is divisible by 6 and 6 is divisible by 3 –> Then 12 is also divisible by 3!
When the GRE tells you that a number n is even –> this implies what?
Every even number is a multiple of 2! So n is a multiple of 2
X is divisible by 3 and 10. Is x divisible by 45?
Given the information about x, we know the following prime factors: 3, 2 and 5.
Break down 45 in its prime factors: 3,3, and 5.
45 Could be a divisor but we don’t know for sure because to be sure, x must contain all the same prime factors as 45 contains!
