GRE Math Basics: Quick Tips
Prime numbers
2 is the smallest prime number
2 is the only prime even number
1 is not a prime
3, 5, 7 are the only three consecutive odd integers that are each a prime.
Digits
In the integer 5,432, 5 is the thousands digit, 4 is the hundreds digit, 3 is the tens digit, and 2 is the units digit.
In a multiple-digit number, there are 10 possible units digits (0 - 9). In the first digit, i.e. the digit farthest to the left, there are only nine possible digits. A zero as the first digit would not be valid. For instance, 0,453 is not a four-digit number. It’s – if anything – the three-digit number 453.
Divisibility Rules
3: Add up the digits in any number. If they are multiple of 3, then the number is divisible by 3. For example, 258 is divisible by 3 because 2 + 5 + 8 is 15. And 15 is a multiple of 3.
4: If the tens and units digits of integer X is a multiple of 4, then integer X is divisible by 4. For example, 364 is divisible by 4, because 64 is a multiple of 4.
5: If a number ends in 5 or 0, it is divisible by 5.
6: If the rules to divisibility to 2 and 3 are fulfilled, then a number is divisible by 6. One hundred and eleven is not divisible by 6. Two hundred and forty six, because it is divisible by both 2 and 3, is divisible by 6.
9: If the sum of the digits of a number is divisible by 9, then that number is divisible by 9. For example, adding up the digits in 4,536, we get 4 + 5 + 3 + 6 = 18. Because 18 is divisible by 9, the number 4,536 is also divisible by 9.
Odds (so to speak) and Ends
Zero is an even integer
Integers can be negative numbers (-3, -6, -17, etc.)
0 \not = 1
x^0 = 1
The problems below can be solved by referencing the above. Good luck!
Practice Problems
Q1. What is the smallest integer that can be divided by the product of a prime number and 7 while yielding a prime number?
(A) 7
(B) 14
(C) 24
(D) 28
(E) 35
Q2. The sum of five consecutive even integers is 20. What is the product of the median of the series and the smallest integer in the series?
(A) 12
(B) 10
(C) 6
(D) 4
(E) 0
Q3. Which of the following integers is NOT divisible by 3?
(A) 624
(B) 711
(C) 819
(D) 901
(E) 969
Answers:
Q1. D
Q2. E
Q3. D