Which of the following is NOT prime?
Select the correct option
A) 1,556,551 B) 2,442,113 C) 3,893,257 D) 3,999,991 E) 9,999,991
3,999,991 = 4,000,000 - 9 = (2000)^2 - 3^2   So this number is a product of two numbers other than 1 and itself. This means it is not prime. This must be the answer.   Let's take a look at (E) too.   9,999,991 = 10,000,000 - 9   Here the difference is that 10,000,000 is not a perfect square. It has odd number of 0s. So this would be a prime number.   Answer (3,999,991)
<result>D</result>
Which of the following is NOT prime?