vinay1983 wrote:If X is a positive integer, is X!+(x+1) a prime number?
1. X is less than 10
2. X is even
Target question: Is x! + (x+1) a prime number?
Statement 1: X is less than 10
To make things easy on ourselves, we'll try small values of x
x = 1: we get 1! + (1+1) = 3, which is
prime
x = 3: we get 3! + (3+1) = 10, which is
not prime
Since we cannot answer the
target question with certainty, statement 1 is NOT SUFFICIENT
Statement 2: X is even
Start with small values of x
Try x = 2: we get 2! + (2+1) = 5, which is
prime
Try x = 4: we get 4! + (4+1) = 29, which is
prime
Try x = 6: we get 6! + (6+1) = 727, which is prime??? TOO HARD - SKIP IT
Try x = 8: we get 8! + (8+1) = 8! + 9. DO NOT EVALUATE! Since 8! and 9 are both divisible by 3, (8!+9) must be divisible by 3, which means it is
not prime
Since we cannot answer the
target question with certainty, statement 2 is NOT SUFFICIENT
Statements 1 and 2 combined
When x = 2, x!+(x+1) is a
prime number.
When x = 8, x!+(x+1) is
not prime .
So, the combined statements are NOT SUFFICIENT
Answer =
E
Cheers,
Brent
